Contractions like "ur", "lol", "k", "l8r" and such.
You're not in a chatroom. This is a forum. I'm sure it wouldn't kill you to type TWO MORE CHARACTERS.

I don't mind people who use abbreviations to lend character to their posts. I say "'cuz" and such a lot, because that's how I talk in real life. I have no problem with that. But every time somebody says "r u..." I get this lovely mental image of a fat, ugly eleven year old with no friends sitting alone in the glow of his monitor deep in the basement.
ARE you a fat, ugly eleven year old? Then don't talk like one.

And while we're at it, MISSPELLINGS. Firefox 2.0 and up have a built in spell-checker. USE IT. If you're one of the poor, sorry bastards using IE 7, they've got a plugin to do that, too.
If you use IE 6, you have my pity.

I also hate newbies who take it upon themselves to correct absolutely every single detail in everybody else's posts, just because they want to prove that they know what they're talking about. Mostly because usually it's exactly this sort of people who have no fuggin' idea what they're talking about.

EDIT:
Oh, and the politically-correct police, who jump on you for criticizing somebody's ideas. Just because you have an opinion doesn't make it valid. "Opinion" isn't a get-out-of-thinking-free card.