I think it's pointless to complain about subject matter. Writers should be encouraged to write what they wish to write, not what the majority wants them to write (espeically if they're not getting paid haha).
Sure, grammer/spelling issues, bad plot devices, inapporiate guessimates etc. are great to post about. Noting that sort of stuff really could help writers improve. But saying you're personally grossed out by pedophilla, or scat play, or yaoi or whatever other kink doesn't help.
Anyway.
One of my pet peeves is with crossovers. I rarely read them because most of them turn out to be the author wanking over one fandom and using the other as a captive audience. For example, in one fic it was a super hero universe crossed over with a demon/demon hunting universe. The superheros ended up cowering in awe, all impressed with the demon hunters (who were disguised as mere superheros) while the hunters went off after OC demons all by themselves. One sided stories are quite annoying to me.