Slice of Life (gag):
Too many have the same jokes that are often too similar. Life sucks, ha ha laugh. A lot of the time they just seem lazy and they don't really try to A) talk about why B) find a resolve for it C) try anything new with it that someone else probably did. The art often seems the same being overly simple and often times uninteresting. A slice of life is generally about character studies and most times people find it as a means just to talk about how rough their life is (not to say I'm not in the same spot with mine). I don't sub to many, and if I do it's usually because the art is intriguing and the characters are actually enjoyable.
Fantasy/Sci-Fi:
Over complication for something that is better left simple. I don't need to know every detail of how the magic system works especially if A) it's never going to be utilized or B) you're just going to make $&%# up as you go just to write yourself out of a hole. I do like these genres, but sometimes writers over complicate things for the sake of those fans who forcibly want answers for something that should be put simply. Let them be fun or impactful, not boring and annoying, I don't want to feel like I'm going to be tested on this.
Thrillers/Mystery:
Goes back to over complications, and it's a shame cause a lot of these ones are good. I just don't have the attention span to keep up with them outside a movie going experience. For me it doesn't keep my interest in a web comic format only because of the amount of things I do read, plus school, plus work, plus family it's just hard to remember what happens weekly... (Unless it's a completed series and I can just read it in one go).
Comedies:
Obvious answer, it isn't funny/interesting. There's multiple cases in which a comedy doesn't resonate with me.
1. Too many inside jokes or meme jokes. I'm sorry I'm old or not in the in crowd, but the purpose of a comedy is to be able to entertain. It's like when my kid tells me a random joke from a youtuber, I just don't care it isn't funny.
One joke, if the entirety of a comedy is based on a single joke ex. The character is fat, then it's clear that the writer is mostly going to write jokes that involve fat jokes and will later forcibly come up with any random thing in order to switch it up.
Shock value! Often times using a means to surprise me to make me laugh, which can work if correctly executed, but awkward when poorly done... It's why there's so many crappy straight to video parody movies, it isn't funny you guys are all hacks.
Comedy imo is probably the hardest genre to write and for those who can do it I promptly applaud your abilities. For me it's easy to explain why something doesn't work for a drama cause there's all sorts of ways to say why it doesn't work that doesn't involve directly hitting the writer, but with comedy it's just hard not to say "It's bad, cause it wasn't funny."
BL:
There's nothing wrong with these stfu, let people enjoy their interests.
I don't hate these genre's (I don't have a specific genre I hate) I just have a hard time getting into them and are often times the reasons I won't sub to some series.