One reason why I left was because this game has a very WoW mentality. Light gear for healers, Med for non tanks, and heavy for tanks. I'd like to be able to wear whatever I pick up, and customize it how I'd like it to be.
Second, I don't like the fact that for alot of Heroics and raids, you have to have a well balanced group or you fail (1 healer, 1 tank, 2 DPS). If I have 3 helaers and two tanks on at level 50, they can't complete things because they are not DPS heavy. MMOs continue to try to have things balanced. Well guess what, in the real world, especially in combat, you don't always get balance.