Gah, this is a good one. I think I'll only do Avatar, because it's the fandom I read/write in most.

1) Extra bending powers.
2) Character bashing.
3) Movie wank in the fanfiction (I've seen it. It's annoying and not funny.)
4) Zuko is a saint and/or Aang is a little boy. Gah.
5) Katara being forced into marriage with anyone against her will. I highly doubt that would happen.