Oh, wow... Lemme just rant for a while.

Illiterate people -- I understand that some people on the internet are new to English, and I respect that, but I can't stand it when people type anything along the lines of: "0MG!1! LYKE I LUFF [email protected]#[email protected]#[email protected]#one!1! KUWAAIII~~``~!!~~~~" Plus the overusage of random Japanese phrases...
Taking credit for someone else's work -- Pretty straight forward...
When annoying fans ruin something -- For example, the Twilight series is immensely popular. I know of a few people who are afraid to get too into it in fear of becoming or seeming like one of those rabid fans. I mean the people who are so completely obsessed with it that it's actually unhealthy; "When I grow up, I'm gonna marry a sparkling vampire who only has one facial expression and we're gonna live happily ever after!!!"

And lots of other stuff I'm probably leaving out at the moment. Okay. I'll shut up now.