Not much from the game, but it's mostly the players.
2) "anal" trade spam.
3) Terrible misspelling that looks nothing like the word it's supposed to be.
4) Over enthusiastic players that "GZ" every single achievement.
5) PvP'ers that think your Arena rating defines you as a person.
6) Players that do nothing but talk about loot, and their ranking on WoW Heroes.
7) Racism, sexism and homophobia. It's not cool/clever/funny/etc and never will be, just makes you look pathetic to someone that has standards.
There's more, but mainly it's certain types of player that annoy me :P