So, I've previously published a guide under this same name, but since then, I've played with Maokai a little more and I found that the build didn't work as well as other tanks in the game, nor did it fully utilize Maokai's damage potential. Instead, it hit a middle of the road that fulfilled both roles, but did neither one very well. So, this new build is to help out his damage output as well as some of his major shortcomings, mainly CDR, mana, and mobility.
So, without further ado, here's my build and how I play the Twisted Treant.

Note* After reading some of the comments and thinking for a few minutes, I decided to add a second build that is more oriented around a 'Pure Tank'. The reason I didn't do this in the first place is because it took me a minute to realize that not everybody plays Premade x.x
So, I've reiterated the purpose of my First Build as well as adding a Second Build that will help with those in Solo Queue who are expected to tank with Maokai as well as those who play him as a Pure Tank.

Build 1 focuses on a more tanky DPS approach to Maokai, allowing him to get lots of damage out while being to take a lot of punishment. I find this to be most useful whenever I know that I have another off tank or a player who's going to build pure tank.
Build 2 is for when you want to go pure tank mode and be able to walk into the middle of the fight and be able to withstand a lot of damage while protecting your team.
The metagames of each build are identical, so it's truly up to you to decide which build is better in the situation.

The number one rule in teamfights is to never focus the tank.
The reason behind this is that the tank is the most beefy character on the team and therefore, is not going to die easily and they just don't do enough damage to make the focus worth it.
So, now the question for us becomes, How do I build a durable champion that is able to withstand the focus of the entire enemy team for a significant period of time, and How do I get the enemy to actually focus me instead of my Carry?
The answer is to draw upon the real life resemblance. In real life, a tank is a powerful piece of artillery, able to take a lot of abuse, all while carrying the biggest gun on the battlefield.
So, this build focuses on building up 'invisible' resistance like armor and MR as opposed to stacking health items like Warmog's Armor so that you look squishier than you really are, all while keeping your damage output high so that they enemy HAS to focus you because you are dealing too much damage to be ignored.

I'M NOT GONNA DIE AND NEITHER IS MY TEAM!!!!
This will ensure the maximum time alive in teamfights, which equates to more time that you are helpful to your team by throwing out Sapling Toss to deal damage, mitigating damage through Vengeful Maelstrom, and keeping the teamfight firmly in your team's hands through using Twisted Advance and Arcane Smash.

Greater Mark of Magic Penetration- This is a great mark that helps get through magic resistance and increase your damage output. This is really important because a lot of Maokai's damage can be mitigated through buying MR. Another viable mark would be Greater Mark of Health.

Greater Seal of Armor- This is a great rune that I wouldn't give up for any other. Its mitigation works well not only against basic attacks and physical abilities from champions, but it also helps reduce damage from minions and turrets, which can sometimes be the difference between a successful attack and a death because of minion focus. Other acceptable seals would be Greater Seal of Health, Greater Seal of Magic Resist, Greater Seal of Replenishment, or Greater Seal of Vigor

Greater Glyph of Magic Resist- I use this because once again, it's a great bonus that you don't have to pay for. This will really help shut down mages in the early game because even with their metagame of Greater Mark of Magic Penetration combined with Archaic Knowledge , you are still left with about 19 MR, which equates to about 20-25% damage reduction. Other acceptable glyphs would be Greater Glyph of Replenishment or Greater Glyph of Ability Power.

Greater Quintessence of Health- I use these because they give 78 additional hp, which is quite a bit in the early game and can mean an extra 2-3 basic attacks with your starting armor bonus. Other Quints that I would advise would be Greater Quintessence of Armor, Greater Quintessence of Magic Resist, or Greater Quintessence of Movement Speed.

Note* I don't really like per level runes, especially for defense. I base this prejudice from this logic:
A flat armor seal, tier 3, will get you 1.41 armor from level 1 to level 18. Now, compare this to per level armor seals. One per level seal will give you .15 armor per level. This equates to an equivalent armor value (1.5) at level 18. Which means that in the early game, when the metagame is going to play the biggest role, you're significantly weaker and later in the game you are just as strong. So, in the end, it's just better to take the flat seals so that you get the biggest bonus when you need it most.

With the newer masteries, I focus around building up as much movement speed as I can to help counteract his severe lack in mobility. So Initiator and Swiftness are almost mandatory. Other than that, I take standard damage reduction all the way down the Defense Tree.
On the Utility side, I take Good Hands for a reduced death timer and Transmutation for a small little bit of regeneration to health.

I elect to take Good Hands over Expanded Mind and Meditation because the large amount of mana from the older Expanded Mind allowed for extra poke from Sapling Toss or an extra Twisted Advance if it was needed. However, with only 12 now at max rank, the mastery points aren't worth the effort. With Meditation so firmly ingrained inside of it also, it makes it a much less attractive option; especially with a Faerie Charm doing the exact same thing.

I use:
Teleport- This helps with Maokai's mobility problems as well as helping you push down lanes or pick up a fallen lane to save a turret. It's also a really nice tool to teleport to a Sight Ward so you can make an effective gank from across the map or get quickly to a teamfight. Overall, a very useful spell.
Ghost- This is a good escape tool, chasing mechanism, or in emergencies, get back to the turret NOW ability.
Other Spells:
Flash- This is a very viable spell for any champion, however the blink is really short and for the most part, Maokai doesn't really have the speed to make use of the distance made. Overall, Ghost is a better option ( Flash is also about to get a nerf to its range, decreasing its use even further). If you choose this spell, change the mastery from Haste to Good Hands
Exhaust- This is a very good spell in terms of Utility. It's great to slow targets and lower their damage output, thereby saving you or your lanemate, or letting you get the edge in a DPS fight.
Clairvoyance- This is a good spell, however I don't really like the hit-or-miss effects of it, so really, it's personal preference.
Clarity- This really helps with your mana problems, however I don't usually get it because I prefer to have the mobility of Teleport.
Heal- This is great if you are laning with a really squishy champion who isn't planning on buying any survivability items for the early game. Other than that, I prefer mobility.

Faerie Charm and lots of Consumables- This is a great starter item because it gives Maokai mana regeneration, as well as builds into his next item. I also like it because it lets you get a variety of Consumables. The most common ones that I get are a Health Potion and Mana Potion to increase my lane survivability and a couple of Sight Wards. This leaves you with an extra 70 gold to either save, or use to buy an extra pair of each potion.

Philosopher's Stone- This gives Maokai everything he could ever need in the early game as far as regeneration goes as well as a very reliable gold income that helps you through any game, regardless of how well you are performing. Normally, when I get earn 620 gold, I go ahead and recall, buy this item, and then use Teleport to get back to my lane. It's that important.

Boots- I generally get Mercury's Treads because they give CC reduction, which is very important to a tank, as well as cheap MR. This will dramatically increase your durability against mages, especially since by now they have normally bought Sorcerer's Shoes. Other boots that I would get would be Ninja Tabi against an AD heavy team or Boots of Swiftness if I decide that I can help my team more by roaming. Sorcerer's Shoes are also an option if you decide to focus more on your damage output than tanking.
Note* If you decide to get any boot choice other than Mercury's Treads, than within your next 2 purchases, upgrade your Philosopher's Stone to Eleisa's Miracle instead of Shurelya's Battlesong so that you still get Tenacity as well as Regeneration.

Defensive Item: The next few items that you buy will be situational and entirely dependent on the enemy team's composition. Each item grans a Defensive statistic, either Health, Magic Resistance, or Armor. The mark of a good tank is knowing when to buy which item and in which order. I personally choose these items as the ones that I use because in addition to granting large bonuses to your defense, they also grant massive Offensive bonuses. I usually buy one or two of the following 3, and then upgrade my Philosopher's Stone to Shurelya's Battlesong or Eleisa's Miracle, but there are games when I get all 3 and never buy Shurelya's Battlesong. It all depends on how the game is going.

Abyssal Mask- This item gives you a massive boost to your Magic Resistance, making you much more durable against mages. It also gives you a considerable amount of AP and its aura makes it so that you lower opponent's Magic Resistance. This is great because now, not only are your attacks doing more damage, but the magic based damage of all of your ally's abilities are doing more damage too.

Zhonya's Hourglass- Increased Armor means that I take less damage from physical sources, making me more resistant to champions and turrets; which is great for pushing because with enough armor, you don't need minions near to take the turret's blows. As long as your team is around, you can take the damage and kill the turret and walk out alive. This is also a good item because it gives a huge addition to your AP, meaning that your attacks now pack more punch and it gives you a pretty awesome active that can sometimes be a lifesaver; or even better than that, charge into the enemy team with Twisted Advance and active Zhonya's Hourglass so that they waste all of their CC and high damage abilities on an invulnerable tree as opposed to your Carries.

Rylai's Crystal Scepter- This item gives you health, which is good against almost any opponent as well as making your attacks hit harder and your abilities slow, which is good considering that you're one of the slowest characters in the game.

Lich Bane- The item is really great because it gives you all kinds of bonuses to your stats as well as adding a lot to your DPS. Overall a great item
Note* Thanks to Antique for the suggestion! +1 Rep

Shurelya's Battlesong- This is a great item because it finishes off Maokai's regeneration problems as well as gives him and the entire team a minor Ghost effect for a few seconds, which is great for retreating as a group, ganking in between lanes, or getting back to the turret to defend.

Sapphire Crystal, Health Potion, and a Mana Potion- This is to help with your mana early game as well as help build into your Catalyst the Protector later. Now, you can also start with a Ruby Crystal and get a bonus to health, but I generally don't do this because it uses all of my gold and leaves me no room for potions. But this is a personal decision and it's completely up to personal preference.

Catalyst the Protector- This REALLY helps with your lane survivability by giving you a bonus to regeneration every time you level up. It also gives you a bonus to your health and your mana, making you more of a presence while you're around. It's also really great because later in the game, you can build it into a Rod of Ages for a more damage oriented tank build or have access to a HELP! I NEED MR NOW! item by letting you upgrade to a Banshee's Veil for a cheap price if you need to.

Boots- Boots are situational and entirely dependent on the enemy team and your/your team's needs. Most of the time, I get Mercury's Treads because they give me early game MR as well as much need tenacity. However, against an AD heavy team, I build Ninja Tabi and if I need to constantly keep on the move, I get Boots of Swiftness.

Defensive Item: These next 3 items are situational and dependent on the enemy team. Each one is designed to grant you the largest possible addition to your stats while maintaining a relatively cheap price.

Thornmail- If I need armor, I opt for this item because in addition to building up your damage mitigation, it makes it very costly for any attack damage based champion to try and attack you. This leads to longer life and more time that you can deal damage and throw down CC.

Quicksilver Sash- This is by far the best, underutilized item in the ENTIRE game. It's cheap, gives a huge bonus to MR, and it gives you a Cleanse effect on a short cooldown. This is great for magic heavy teams or against champions that keep trying to keep you out of the fight by CCing you.

Guardian Angel- This is one of my favorite items because it gives a lot of bonuses against all damage sources as well as a great passive that prolongs your use in teamfights. This is normally the best item to get when you're going up against a pretty balanced team.

Lich Bane- This is to keep your damage output high while also giving you a little bit more MR, and helping you run around the map.
Note* Thanks again to Antique for the suggestion

Guardian Angel- This is one of my favorite items in the game. It gives massive resistance to all damage as well as prolonging your use in teamfights by bringing you back to life after you die.

Thornmail- This is great for shutting down "Right-Click Wonders" like Tryndamere or Ashe.
Note* This item deals damage based on 30% of the damage that they did to you, so if you get this item, start building Health instead of Armor, so that you can last and they can deal more damage, thus returning more to them.

Sunfire Aegis- This is a great item for Health and Armor. It also has a very good passive that makes your presence in teamfights even more felt.

Quicksilver Sash- This is a very cheap item that gives a LOT of MR as well as a Cleanse effect for an active. Overall, this is probably one of the greatest items in the game and is very rarely used.

Sunfire Aegis- This is a great item for both Health and Armor. This is a good choice against teams that need extra AD mitigation and when you need a little bit more DPS in the teamfight.

Randuin's Omen- This is another good item choice for Health and Armor. This is a good choice against teams that need extra AD mitigation and for when your team is having a really hard time taking the enemy team down because of the active.

Warmog's Armor- This is the best Health item in the game for when you're having pretty free reign over minions.

I focus most of my level-ups around Sapling Toss because it has the highest damage potential of all of your skills. Next, I prioritize Twisted Advance because as it gets higher in rank, the root lasts longer, which means that you or your team have more time to kill them. Next, I rank Arcane Smash for the very reliable damage output and slow. I take Vengeful Maelstrom at levels 6, 11, and 16.

Note* If I find myself doing more saving than damaging, then I re prioritize by leveling up Twisted Advance first, Arcane Smash next, and finally ranking Sapling Toss last.

So, here are a few of the combos that I generally use with Maokai:
Use Twisted Advance to get close to the target and deal damage, next use an auto attack to proc Lich Bane. Then launch a Sapling Toss on top of them and activate Vengeful Maelstrom so that you and the enemy are standing just inside the nearest edge to your base and then use auto attacks until just after the root time on Twisted Advance has worn off. Then use Arcane Smash to deal more damage and slow so that you can walk up and auto attack again to proc Lich Bane. Finally, just before they walk out of the radius of Vengeful Maelstrom, activate it to deal more damage.
Note* If you have Rylai's Crystal Scepter, then this will probably slow them down enough that you can walk up and use another auto attack and proc Lich Bane again.

In the laning stage, one of your main jobs is to be to protect your lanemate. So if the enemy is going for them, I use Twisted Advance to keep them still so that they can get away and then, just after the root ends, I use Arcane Smash to further slow them and deal out some more damage.
Note* If the enemy doesn't have a lot of health left, then using Sapling Toss on top of them after you use Twisted Advance may net you a kill in addition to getting your lanemate out safe.

A very quick way clear minions is to throw Sapling Toss on top of them so that they take damage from the impact and the explosion, then walking up and throwing out Arcane Smash laterally into the line to mop up the remainders. This method should only be done when you are either confident in your mana supply, or when the gold that you'll get is worth the mana that you just spent.
Note* This loses it's effectiveness after you get Sapling Toss up to either Rank 3 with about 40 AP, or Rank 5. Then, a Sapling Toss by itself will one shot the back row caster minions.

Very Important*: Be aware of what your enemy can do, especially if they have blinks in their abilities or if they have lower health and are more prone to use Flash. If you use Twisted Advance, and they use that blink, it will drag you with them to where they end; which is good if you're trying to chase. However, if you're laning and they blink back to turret range, then you will follow them into turret range and the turret will aggro on you.

So, thanks for reading my guide and I hope that you enjoyed it.
I also ask that you post any comments (positive or negative) below. I'll keep trying stuff out and keep an eye out for anything new and improved that happens.
In the meantime, please don't downvote the guide because you don't agree with one part of it.
Imo, Mobafire isn't about posting and saying, 'My guide is better than yours', it's about showing other people how to play and exchanging strategy with other players in order to get better.
So, if you do elect to downvote, please comment on the reason why you did so. That way I, and anyone else who looks at this guide, can see your point of view and we can all grow as players.
That being said, I wish you luck on the Rift!!