Edit: Somehow had this scheduled to post next month instead of last week. I guess I don't have to write anything new yet! :P
As a counterpoint to my last post about awesome people, I feel I should point out there are also a lot of jerks out there. I don't know whether it's because the internet gives people the illusion of anonymity or if it's just that there are jerks everywhere, but you will meet them.
I won't name anyone this time because I don't want the post to be about who did what to whom and when. Anyone who gets involved in a fandom will meet up with jerks. Sometimes they're forum moderators who're on a power trip. Sometimes they're just dumb people who are so rabidly involved in their fandom or their particular sub-group of fans that they forget how to respectfully disagree. Sometimes they're just idiots who think that hobby related activity (especially online) means you get to suspend the rules of common courtesy and etiquette.
I don't like Joss Whedon's body of work. I know, jaws have hit the floor all over the internet now. I have nothing against the man; I just don't like most of his stuff. The only exception is the movie Titan AE. I don't like the new Battlestar Galactica either--although I did at first. I've been personally attacked by members of those fandoms because I said I didn't like the shows. (I have plenty of friends who're into Buffy or Firefly or BSG, so I'm not singling out those fandoms. I'm just talking about my experiences.)
I've also met people who seemed to have no interest in reading (or watching) my work except to leave nasty comments. (Which is entirely different than leaving constructive negative feedback) Most of those are anonymous but not all.
The most hurtful experiences I've had in fandom have been with people I considered friends. I've had people suddenly turn on me for something I wrote in a story or said in a fan community that they didn't like. There have been people who "got bored" or left a fandom for some other reason and seem to have decided that my friendship wasn't important anymore.
What I've learned is that you need a thick skin to be involved in fandom, and an even thicker one if you plan to create fanfiction or fanart of any kind. It's great fun, and most of the people you'll encounter are wonderful, but you have to be prepared for the occasional jerks. Try not to take their actions personally, and try to avoid engaging an argument wherever you can.