To comment on something said much earlier about bisexual characters. Reread a lot of fics here. In fact, it is something, that and yuri to be honest, that will get me to absolutely lose interest in a fic unless it is done incredibly well.
Almost everything I hate has already been said, but here's a new one: not properly developing a romance. I can't really criticize one shots too much about this, because of the fact that they are one shots, but whenever I see a fic that has an alt pairing and they are going "ooooh, true love" off the bat is absolutely infuriating.
You see it fairly often with ship fics; fics that have a specific pairing, work off of it, but don't explain the how and the why. Examples would be most alt pairing fics in AtLA (hell most fics there in general), Donut fics in XxxHolic, and even the Watauuko or however that pairing is supposed to be named. Yes, I know about the subtext for Donut, I've seen it, I read the series, but that doesn't mean you can get away with not developing what is subtext into something more. I also don't mean "Hey, I suddenly realized I'm gay/in love with you," I'm talking about full on romance, m'kay. You need to build up on it, add some sexual tension, doubts depending on relationship statuses or other things. It is possible to use a bit of bullshit to get the ball rolling, but to keep it up, you need good, solid writing.
More fun with romance: rehashed plot lines. We've all read this or that plot line, and yeah, I know how hard it is to come up with something new sometimes. However, try to think about tweaking ideas. Instead of the same events and then the change, what about changing events leading up to the big moment that was already used? Or what if this character came to terms with things before that. This is mostly aimed at Love Hina's fandom, but I am extending it to Eva as well.
Don't add bullshit interpretations. I could go on and on about this, but quite frankly, it could easily be solved if people actually bothered to write the actual characters rather than give another character the same skin and name and call it good. Now, it is one thing to have a character change for the sake of an AU (sometimes, there are certain bounds I have on that), but claim it's a specific character and then have them do completely the opposite of what the character would do for no reason that makes sense.
EDIT: actually, there are a lot more, but that's because I am a twig like mass of hate and mild rage.