I'm not deeply involved in fandom. Being on the fringes, for the most part, has a positive impact on my experience. I'm not very active on the boards, but I read a lot of them.
What I don't like is seeing the negativity, especially towards characters. I strongly suspect that my dislike of Lindsay on NY came from this site. I doubt I would actually care enough without people pointing out how annoying she is all the time. I probably wouldn't notice some of the annoying quirks of the shows if no one was pointing them out. I'm a little further removed from the Criminal Minds fandom, but the negativity there, and the fierce loyalty really do turn me off. After any given episode, there is so much criticism from the so-called fans.
I used to see more of that here, but I think the CSI fans have mellowed somewhat over the last couple of seasons (vegas, at least, I don't really keep up with NY and I rarely watch Miami). Either that, or I've gotten better at avoiding the worst of the threads.
I think it's interesting what everyone says about the spoilers-- for me, spoilers enhance viewing. I always have to watch any show (or read any book) twice to get the full effect because the first time through I'm so wrapped up in the plot and what happens next that I miss the things I actually love. So the more I know going into the show, the better I like it.