I find it difficult to make any fandom friends because of this issue, actually. So many of the fandoms for the anime/manga/video games that I like act like crazed 12-year-olds who can't see anything beside their favorite ship or who's so omgsexyhawt.
Maybe I'm just a fuddy duddy, but I'd like to just make friends who I can talk to about the stuff I watch, read, and play without having to worry about pissing them off if I don't like their favorite character evar.
Dunno. I watch Sword Art every week and I see topics about it everywhere, but nowadays, I don't even reply to them because they are all the same. Fanbases aren't that bad. If they annoy you, just ignore em? Watch the show, don't discuss it with the fans. Not too difficult (at least for me).
The thing is, it isn't just the forums. For example there was the Anime Convention a month ago in my country and the SAO fans were wrecking the SAO booth and everything was sold out in less than 3 hours. SAO fans went crazy there and a lot of people were really upset about it.
Raging, pre-pubescents had moderators shut down the Shugo Chara fan page on Crunchyroll. Needless to say, I was angry.emotion_facepalm I also think they're the reason why the anime continued after the manga ended. Everything became poorly animated, filled to your throat with sparkles and pointless characters. It really was ruined by episode 80.
Then I see an anime with an annoying fanbase, I just avoid talking about it on the forums/in conventions/etc. I do like discussing the series I like, though... so yeah, the fanbase might make my experience slightly less enjoyable. But I'm not completely warded off from a series because of it.
On a slightly related note, I'm a bit annoyed by some yaoi fangirls when they get all rabid. e.e
It doesn't bother me too much! I watch the anime because I like it and I'm interested in it and of course there's always going to be those people that are just a little to enthusiastic about stuff but It doesn't bother me!
For me, yes it is the fanbase that drives me away from the anime/manga and they makes it difficult to pick back up. For example like Naruto, read it in middle school when it first came out and I liked it and now, I don't even catch up with the latest chapter; The extreme fans are always like "OMG! NARUTO THIS! SASUKE THAT!" And it's a lot worst when the extremists are weeaboos. They ruined ninjas for me.............stare