I can't say I've ever been actively deterred from playing a video game by it's fanbase, but there have been a few occasions where a game's fanbase has lured me back in. For example; I hadn't touched Final Fantasy VII in years before I learned that a group of dedicated fans had since broken the game down to a science and taken the gameplay to an entirely new level. Enemy-Skills that I never knew existed, applications for Limit Breaks I never thought were useful and self-imposed "challenges" (Minimum Level Cloud Only, Command Materia Only, Initial Equips Only, No Materia) added layers of gameplay that I simply didn't know where there before discovering FFVII's active fan community.
Sure, there's a lot of negativity to be found. As obsession goes, those fans could have a fair shot at being called some of the most rabid. Still, I'd be lying if I said I didn't learn anything useful from them.