Mary Sues. They have no flaws, they always have to win every argument and every male character MUST fall in love with them. I don't take canon too seriously, but the character I play doesn't seem to be too swayed by immaterial things such as heavily sequined dresses. As far as HP goes, they usually seem to be Parseltongues, Animagis, Metamorphmagis, Legilimens, Occlumens, and can do magic without a wand. "that is right, oh great one, Go destroy You know Who... oh wait, of COURSE you're not afraid to use his name! What was I thinking?" I could go on, but we all know what Mary Sues are.
Post Whores. They have to be in every single damn plot imaginable. (often the Mary Sues do this). They cannot have something going on without them being in the center of it all. Often they must be the hero.
The Poachers. Yes, people who join your RP and then invite half the group privately to their own group. Personally I think this is a selfish act. I had gone to great lengths advertising my board everywhere, and this Yahoo comes and PMs them to leave ours and join hers. Kudos to the majority that remained.
The UNcanons. I don't care if your character goes a little out of character, for example, you like Slash. Work it in well enough, great. It's seeing a character like Severus Snape or Lucius Malfoy crying their eyes out because a woman (Mary Sue! I guarentee it!) refused to accept their proposal.
Some of you may tell me I have no life for finding these annoying. Those who say so will be instantly deemed Mary Sue of the Day.