Maokai is probably one of my preferred tanks; he has great crowd-control potential, uncanny survivability in teamfights, can be a force of great disruption along with some decent damage, and he even tosses in a 20% damage reduction for he and his allies within a zone, ending in a nice AoE nuke. Here are some tactics I use when playing this furious tree:
- Try smart-casting your Q. It'll essentially turn it into a quick smash in the direction of your cursor, and may help you quickly land the slow/knockback following your other moves.
- I normally level my Q first; the damage output (combined with the slow) will help you completely dominate, and if you're in a lane, your carry will greatly appreciate the utility and short cooldown. From small skirmishes to teamfights, your Q will be a hard-hitter to multiple targets, often followed up by a Sheen. If you're experiencing difficulties in lane, your E is an alternate avenue to level first, as it will help you zone your opponent. It's all really just a matter of your preferred playstyle. Personally I like to get up in my opponents' faces mashing my smartcasted Q and knocking them around until they die.
- When above 30-50% mana and fighting an opponent who has committed to a little 1v1 action, you'll often have an urge to toggle your ult on, and then off to deal damage. Try to avoid doing this. Let the ult stay active for as long as you can safely support it with your mana. In order to better your chances, you'll want the maximum possible damage output before you switch it off. Obviously you'll also take less damage for the duration. Try to position it halfway between your current fighting area and their likely escape route, and be weary of summoner spells such as Flash.
- Your passive is godlike. Incorporate a fair amount of health into your build, as well as the standard resistances. Rod of Ages is a great item for Maokai, as well as a Banshee's Veil. The Iceborn Gauntlet was built for Maokai; try to build a Sheen earlier on and upgrade to this form asap. Not only will you be tanky, you'll be even more disruptive, and in any teamfight you can practically expect every melee attack to restore 7% of your max HP.
- In the laning phase, make use of your passive whenever it is up. Your sustain is incredible.
- Use saplings to harrass, or to temporarily scout the jungle routes in the event that you don't presently have a ward down. You can also use them to check areas you're unsure about. In combat, throw them at your target before using your W; if you time it right, you'll hit them for complete damage from both spells, and they'll be unable to move when the sapling explodes. That's when you can hit them with your Q for added pain.
- If you EVER get this champion in the ARAM queue, don't give him up lightly. He was built for teamfights; the more the merrier.
That's pretty much all the main pointers I can think of. Maokai is incredible amounts of fun to play, and he has everything you could want in a tank. If you've not played him before, I strongly recommend it. :-)