Great thread. This is my first post here, and I was intrigued by others' comments. This is going to make me sound like a snob, but it took me a long time to read any fanfics (and write any!) because I got so turned off by a number of things. Let me be long-winded here: *grin*
1) Bad spelling, grammar and story structure. UGH, I won't even read it if this stuff persists past a simple slip or two. Internet shorthand is the devil.
2) Summaries that read: "I don't know, just read it." If you can't think about your story long enough to come up with a good summary, I can't waste time reading it to find out what it's about.
3) My stupid (and narrow-minded, I'll warrant) pet peeve is A/U fics and stuff that would have been unlikely to occur in the original work (ex: unlikely couple pairings, etc).
I'm aware that there are some stellar fics that happen to be A/U, well-written in their own right, but if you want to pair two disparate universes and make unlikely things happen, I'd rather you do an original fiction. That's just me being snobby there. I just feel that the original writers or producers of these works spent a lot of blood, sweat & tears on them, and their body of work deserves to be respected the way it was intended. It's for that reason that, so far, I've only written fics that fill the "holes" the original works skipped over. I know. I'm obnoxious...and partly hypocritical, considering one of my works is unintentionally A/U. I started it before seeing one season of the TV show, whoops. So some minor things that shoulda happened...didn't. Erk. (Yes, it's now a personal thorn in my side. Color me obsessive.)
4) I gotta go with those of you who dislike songfics. They pull you completely out of the story! If a writer wants someone to have a certain song in mind when they write a scene, I'd rather s/he puts the name of the song in an A/N, or mention that it's playing on some stereo in the story, rather than keep jerking me out of the action with a line or two at a time. Bleah.
An interesting discussion, guys!