Portal fans. They really made the game less enjoyable when you couldn't avoid trilions of jokes directly take from the game on the internet. Slightly brought down the humor in an otherwise enjoyable experiance.
Otherwise I can't really say I know where fans ruined the game. People mention a lot of multiplayer games here where the player base is rude. But lets be honest, do we really wanna count the rude people as "fans" of those games? Personally in my experiance with Halo it's the really die hard fans that can contribute to a good multiplayer game. Those who have played every halo game since the first one and enjoy the gameplay tremendusly. For me, those people are very laid back and calm when playing multiplayer, cause they have done it for years and know they don't really need to prove anything.
A lot of the dickheads online really doesen't give a damn about the game, they could be playing halo or call of duty or whatever. Way to often when I stumble upon them on Halo I can notice that they haven't touched the singelplayer or other aspects of the game.
Same thing with Warcraft, I played a roleplaying server and the times when I RP with people there it was with people that was big fans of warcraft as a whole. That knew waaaay too much about the lore than should be healthy, and read every quest discription fully. These people were a lot nicer to hang with than the average.
Of course there are excemptions. But I would say that the average "fans" of a game knows how to behave kindly in that game better than anyone else.