I love fanfiction. I am, however, extremely picky when it comes to finding a suitable one to read.
For one, most of the fanfics I read (90%) - and I'm not ashamed to admit this - are romance.
9% are humor.
Every now and then, I will go towards another genre.
I will not read a fanfic if:
If I click a story open and it's filled with typos/grammatical errors (IE the author apprentely didn't want to take the time to bother proofreading and spell checking initiating a very unproffesional manuscript) I will click out IMEDIATELY.
Any story that starts making the characters too OOC, (humor fanfiction or alternate universe fiction is the only exception) I stop right there and look for something else.
The idea of the story has to fit the universe. I would find it hideously rediculous reading a Mario fanfic with bucketloads of gore, swearing, ect.
I do have a limit for OOCness... which means if I were reading a romance in which the characters were SLIGHTLY sappier than they might really be, I'll let it slide. Mainly because I'm quite guilty of it myself in my own fanfics.