1) I guess this one is my bad but when i bought it i didn't realize the world was separated into isolated Tiers. That took out the majority of the fun for me, knowing that it wasn't an actual world. Just a couple of levels.
2)The animations and graphics were... well... Bad. I had the game pumped to the max graphics and it looked like WoW on the lowest setting. I mean, i normally give games shit for always demanding to push the bar on graphical power on computers, aion for example, but to push it so far down that the game on max looks worse then WoW on low...
3)Uninteresting trade skills.
4)PvE was bad. The worst cases were PQ. While great idea in theory they forget the inherent laziness of players. Most people just stood and watched as me and one other person tried to do the public quest. Only helping once we were almost done the last phase to get free loot.
5)Class only gear. I know i know, stupid complaint, but it bugs the shit out of me. I like the idea of light, medium, heavy. Where the armor is in tiers and classes can only wear up to a certain point in the tier. I can't really explain WHY i like it, it just seems more interesting to me.