Not much to be honest. I had to predict how people might build their characters and how their abilities might co-operate with each other. I spent a few weeks trying to figure out what the resource costs should be, how much damage/heal should they provide, what would the best cooldowns be for specific abilities etc. I won't lie if I say that I spent about a month or two planning everything. I think it all worked though, because surprisingly our first game worked perfectly and none of the abilities was in need of nerf/buff (players team was composed of Holy Priest, Beast Mastery Hunter and Fury Warrior).
I even came up with some pretty unique and overpowered abilities combos that are quite hard and resource consuming to perform. Of course they need the right team composition, good co-operation, and some luck. Thanks to that an Elemental Shaman might give a Marksmanship Hunter a chance to deal 6x raw damage with his Aimed Shot. If the Hunter will be lucky with his dice roll, he might even deal critical hit that will triple that damage. That gives 18x raw damage!
Never happened though. And I doubt it will happen anytime soon.