Not really a specific fandom but I hate people who are over-overobsessive about anything, especially Anime. I'm fine with Otaku's and people who like anime in general but when you spend most of your time discussing, watching or obsessing over it then I think somethings wrong with you. In one instance I had friends who stated that Code Geass was horrible as it had copied from the Anime Death Note due to it being released at a similar time, refusing to watch it and bagging anyone who disagreed with their opinion yet when i convinced them to watch it, they immediately started bagging people who disliked the show for legitimate reasons acting as if they hadn't held any negative opinions themselves. I'm no saint and I can be an arrogant asshat at times but at least i recognise my faults.
I also dislike user's of Reddit and 4 Chan who dislike the opposing communities for no reason besides their enormous Ego's and respective superiority complexes.
Phew, got that off my chest.