This is a toughy, since it's hard to narrow it down or really get it to a smaller number. Like...one that's under a hundred. >.>
Well, to start...
I can't understand what people have against spelling out full words during games where you have the time to do so. Seeing things like u y lol and whatnot just piss me off for some strange reason. The same holds true for punctuation.
I also hate stereotypes that people seem to link everyone in said stereotype to. Blacks, gays, furries, Irish, Scottish...they've all got a stereotype that everyone links all of them to. There are exceptions, but people refuse to acknowledge this in most cases.
Then there's griefers/trolls. I don't understand what it is that gets them off when they do the things they do, since all it does is make them look like an idiot. And you can't bash them for it since their conscience doesn't allow them to understand when they're the ones the joke is being played on. They just act like it was their plan all along.
Lesse...there's also how people are trying to fix the gas economy, and yet I still see these gas guzzling trucks, SUVs, HumVees that make up about 90% of traffic now-a-days. It just floors me how stupid some people can be...
Magazines. I've begun to hate looking at the magazines that sit right next to the check-out at grocerie stores and stuff. All this stuff on the front. Scandal! Rape! Marriage! Break ups! How to lose weight in a week! I sometimes feel sorry for the stars shown on these magazines. They can't go outside without a guy with a camera following them, questioning/interrogating them, then twisting their words to make it sound as bad as is possible.
...sorry, I'll stop now.