Okay, strongly disagree with you on the Ruby Crystal start. It is very risky and usually not even worth it unless you plan on getting a kill before level 6. The health it provides you, 150 HP equals to one Health Potion (so, you are walking into lane with 3 pots). It is a lot safer to bring Doran's Shield as it will provide you with more sustain or a Cloth Armor if you feel you're going to get whacked.

Mercury's Treads are NOT core, they depend on the situation. Although you do mention this further down, I still think people might get a wrong idea on when to buy them.

Why don't you take Oppression as a mastery? It benefits Dr. Mundo A LOT when fighting.

I do partially agree with you on Dr. Mundo not having a lot of counters, but he does, however have many, MANY skill matchups that don't start in his favour, mainly because of his poor early game.

"These runes also allow you to use your abilities more often" I don't see how flat stats provide you with Cooldown Reduction. You don't even have CDR Runes there :P

"In some situations it may be a viable option to take Smite" What kind of situations?

I think Pros/Cons should be right after the introduction, not after the Summoner Spells.

Don't think Dr. Mundo is a weak duelist. He is, AT THE VERY LEAST, decent. He can win most 1 vs 1 fights with relative ease, provided you land your Infected Bonesaws.

You can't clear a monster camp with Dr. Mundo without Smite as a summoner spell, MOREOVER starting with Heart Zapper since it drains all the health out of you. You'll die. If you plan on asking your jungler for help, you'll only delay him and leech the EXP. Don't think we'd want that...

Maximum Dosage doesn't DIRECTLY give you fighting potential, but it makes you pretty much "invulnerable" for the duration of the spell, so you are able to outlast your opponent. If you gank a lane and have Maximum Dosage you should be okay.

"or go and lock down their adc with cleavers if you have Thornmail." You should do it even if you don't have Thornmail, you can take the damage. What about APCs?

You don't mention any enchantments for the boots (you put Enchantment: Furor in the cheat sheet but don't mention it in the items section).

For a quick guide to read while the game is loading, it is pretty okay. I believe that you could do a lot better, however. Also, go ahead and play some ranked games, there is nothing to fear: climbing the ladder is part of the experience and fear shouldn't be holding you back. Besides, people tend to take these seriously so you'll find more worthy opponents.