Too many to list. Obsidian's bandwidth would go through the roof if I listed everything that annoys me. But here's a short list.
1- People on my IM list who never IM me or never IM first. So then after maybe a month I finally IM them and they respond with "oh my god, I missed you! You never talk to me anymore [insert pouty sad smiley]!". Um? MESSAGE ME THEN YOU DUMB TWIT. You DON'T miss me, you DIDN'T miss me and you NEVER missed me if you couldn't be bothered to click on my name and send me a IM during that time. What you DID miss was being an attention whore and having someone message you to make you feel like you're the center of the universe.
2- Internet "bad ass". The kind of person, usually in their teens, but sadly some older than that, who go around to forums being intentional assclowns under the guise "lol you take this too seriously. get a life, loser". The kind of person who intentionally disrupts conversation with one line, elementary school insults because he/she thinks it's somehow funny. The kind who then turns around and says "lol if you want a piece of me, come and get me. I live [insert city which happens to be across the freaking world from you]" knowing full well that no one is going to bother hopping on a plane and flying 6 hours just so they can beat up a pimply faced nerd who resembles Napoleon Dynamite.
3- Phony people. People who pretend to act friendly but in reality they're not. People who pretend to like you but in reality treat you like an inside joke with their friend.
I'm sure there's more but those jumped out at me.