Okay, so I've tried most if not all of the Maokai builds posted online; Most of them focus on being Either being pure tank or pure caster. Being pure anything with Maokai is a huge mistake; If you go pure caster, your too squishy, and if you go pure tank you are too (for lack of a better word) useless. With this build you get the best of both worlds; You end up being an okay tank (and a magnificent tank if you are good with his ult), and you end up being a powerhouse caster, with stuns, nukes and slows, and reasonable damage (upwards from 200 per ability) galore.

Well, to be fair, runes can really be basically whatever it is that you would prefer,, but your base Magic Resistance is extremely low, therefore i would recommend some Magic Resistance runes, and as much Cooldown Reduction as possible. Ability Power ratios with Maokai are alright, but focussing on Ability Power is definitely not going to be getting you anywhere. Basically what you want is to be able to spam all of your abilities as fast as possible, and if you can get end up having a high enough Cooldown Reduction, your damage doesn't really matter.
But again, Rune set ups are pretty arbitrary, all the runes help in different ways, i just think these ones work the best for this particular build.

My strategy with Maokai's masteries is that Cooldown Reduction, Mana (maximum and regeneration), Movement speed, and beefing up your Summoner Spells should be your primary focus. If you focus on them, then you don't really have many options with where you can end up putting your mastery points.

Basically, what I have found from using this build is that the order I have the items set up is the most effective buying order. It deals with maokai's mana (again maximum and regeneration) problem right off the bat, then it takes care of his movement, then you can really focus on your defence. I can usually get my full item build by about level 16, I can get it even sooner, by level 14 if I am having an exceptionally good game. Once you've got your Force of Nature and your Frozen heart, your defence is just about as high as it is going to be for the game.. The rest of your defence is through your by level boosts from your rune setup (hence the runes I chose). Rylais scepter is chosen solely for the slowing feature of the attack. When you end your ultimate, it slows all enemies inside down 15%, when you throw your sapling... it slows them 15%. Basically, you can keep all your enemies slowed down the entire teamfight. The 80 Ability Power doesn't hurt at all.
If you don't care so much about having Crowd Control on every one of your attacks, be it a stun or a slow (which i personally believe is one of the best parts of this build), then get phantom dancer for the movement speed, because you won't have the slows to help you chase. If you go the phantom dancer route, you might want to consider a Tiamat instead of a Chalice of Harmony... but this will considerably slow down your item building process, making it so you don't get your full item build until after reaching level 18... which usually I win the game by level 16, so why bother with this variation (In My Opinion).

The skill sequence is pretty simple. Twisted Advance and Vengeful Maelstrom are your bread and butter. once you have your First three spells spells at level 1, focus on Twisted and Maelstrom whenever you have a point available (with priority going to your vengeful Maelstrom over twisted advance whenever it is available). If neither of these spells are available, put it into your sapling toss. Arcane smash is necessary to have, but maxing it early on is not a priority.

ALTERNATIVELY you can split points between Twisted advance Sapling toss and Vengeful Maelstrom for increased farming, but it isn't really necessary with the champion kills you should be getting.

Maokai needs an escape mechanism. without it, he dies. A lot. Ghost works best for this, but flash is good to. If you find when you play this build you have trouble escaping with ghost, try out flash, but if you go with flash, you want to find a way to increase your movement speed, usually a zeal works and then sell it once you need the space.

Ignite can be replaced with exhaust, but A) You already have 4 attacks that either stun or slow. B) Ignite is more effective, as often times you will get your opponent down to 60 health and they will either get backup or have an effective escape mechanism. Pop an ignite and Boom: Roasted.

Teamfights are kickass. If you are having trouble getting kills and are off to a slow start, once the teamfights begin you should be getting an assist on almost every kill in the team fight. My kills usually hover around 12-13, but my assists have been as high as 35. usually a minimum of 10-12 assists per game, even if i'm not getting a lot of kills.