Fanfictions themselves never upsets me. At worst it just makes me go "Bugger this". With that said, I usually try to give some constructive criticism, no matter how bad the story.
What annoys me more within fanfiction circles is when a particular portrayal becomes popular and widespread, and people start mimicking it and blasts others for not doing so, even though it's not canon.
Also the tendency to lionize characters they like and demonize the ones they don't like. Or rewriting the character wholesale for shipping, making the whole point of the character moot in the firsst place.
Good examples?
Ginny Weasley and Pansy Parkinsson.
Ginny: Usually portrayed as the perfect girl, funny athletic and everything(unless the writer doesn't like her) and very nice while still sarcastic and witty. She can do no wrong.
The actual book Ginny had a ton of negative character traits that are glossed over(spitefulness, selfishness and general rudeness- traits that most Gryffindors shared actually, but Ginny was most clearly this way).
Pansy: Becomes a big villain character who everyone dumps on, despite not really being horrible in canon, but not really pleasant either.
Pansy is also a common "rewritten personality character", where people want to ship her with Harry but to do so they don't write Pansy, they write their own OC with a flawless personality, just under the name of a canon character.
Finding balanced version of both characters can be stupidly hard.
So yeah, individual fics don't upset me. Stupid trends within fanfiction circles do.