Anything over 100k words that doesn't involve completely nonsensical same-gender pairings. Sensible ones are fine (I think I've ran into three that were readable!), but the stuff where they take folks in th'original series that blatantly hated each other and throw 'em in bed... makes my brain melt. Tend to just overtly avoid yaoi stuff, because most of what I've attempted to read was just... bad. Bad by
fanfiction standards*. E: Which isn't to say I haven't/wouldn't read stuff that's good, just that I don't bother trying to sift through it anymore
... mostly just a size thing, though. I've been dipping down to 60k over the last year or so, but anything smaller than that tends to... not last very long. Th'100kers like to disappear into my brain overnight, heh. That and basic grammar. Punctuation, yanno'? I've been meandering through fics long enough that properly used commas, periods, etc., are like sunbeams parting a cloudy sky and the world bursting into divine chorus.
Fandoms themselves, I regularly read... uh, a couple dozen? And happily branch in to stuff I don't actually know jack about. Good readin's good reading.
*For folks that aren't familiar with fanfics, take this illuminating quote: "[Finding good fanfiction] is like looking for a needle in a needle stack, only the whole thing is covered in shit
and everything is on fire." Mindless yaoi stuff tends to throw in some arsenic or something to the mix. Mind you, mindless non-yaoi stuff does the same thing, but th'yaoi is somehow more prolific.