I've never entirely liked any kind of obsessive fan. Although they may still be a perfectly reasonable person in all other ways, their obsession always annoys me to no end.
First of all: Yaoi fangirls and Yuri fanboys. See those two that are in completely heterosexual relationships? They're not gay. I don't have anything against those who happen to be homosexual, it's just the blatant disregard for the established facts that gets to me.
B: Fan Fiction writers. Do you have a good idea for a story? Then go write it. I don't care, as long as it uses proper grammar, with the exception of certain characters' particular idiosyncrasies. However, it's taking it a bit far to write established characters' personalities totally differently. Self insertion I can handle, too, as long as it's just a cameo or small role, but Mary Sues are going too far. Just cause you like him doesn't mean he's gonna dump his old girlfriend in a garbage disposal and change his outlook on life just to praise you all day. Kind of ties in with the above.
3: Haters. The ignorant and intolerant people who hate something and the people who like it and never did the research into it. For example, I play Team Fortress 2 a lot. I like it for its bright colors. I don't really like Modern Warfare 2 very much, though. It's too gritty. I don't, however, hate the people who play it a lot. It's the game that I don't like, not the fans of it! I try to stay as informed and as neutral as possible when describing why I don't like a certain game. I list the things that I like, and those that I don't. I can't STAND the people that take everything against their game like a death threat against themselves, their family and their pet dog and reply with the statement: "no ur gay".
It's the disregard and willing ignorance that gets to me. Anyways, I should probably stop here. I'm starting to rant.