What I hate about slash (and since this is a community regarding Silent Hill slash I'll direct my comment to this fandom at this particular moment) is the attitudes of the people who write said fic. The few people I've come across that actively write Silent Hill slash act like it is the be-all and end-all and the only thing worthwhile to come out of the fandom. They get all anal if you don't praise the hell out of them for their particular pairings of choice when most, if not all the time the pairings really don't make much sense or are highly implausible. If I say I don't like a certain pairing, that's my opinion. I'm allowed to have an opinion and if you can't handle opinions, then tough!
Like I said, I do read slash from time-to-time ... but that doesn't mean I have to like every single piece that I read. A majority of the slash I've come across lately (not just Silent Hill and the other fandoms I've listed above) have been so badly written that you just want to headbutt your computer monitor out of sheer frustration.
Last but not least, if I hear one more SH slash writer saying they are going to include Akira Yamaoka in their fic I will personally see to it that they are flamed and email bombed mercilessly.
Oh yeah ... any slash community that lists children in their interests lists are sick and really make my skin crawl.