I'm surprised I didn't bring up some of the suggestions here in my initial post.
First and foremost; censorship. Yeah, it's pretty dumb no matter how you look at it. I mean...it's porn! Censoring genitalia in porn is bad enough...but censoring DRAWN genitalia is just fucking absurd!
And of course, the fat/ugly men. Goes without saying, really. I'm not sure who actually enjoys being met with the unpleasant imagery of a fat slob in the middle of a sex scene with a beautiful woman.
As someone mentioned before, all the talking. Yes, that can get pretty grating...especially when it's coming from a woman with an annoying high-pitched voice that seems prominent in most female characters in hentai.
I also despise the abundance of grimdark hentai. Ones where rape is the central theme of it all, with things usually ending badly for the women, or the "good characters" in general.
Which finally brings me to rape, which I want to discuss just so I can share a specific example. Anyone remember Pretty Pridot by Bingo Torte? The first series of videos (blowjob and titfuck) remain some of my favorite animated porn videos to this day. But then the "main" series happened, which turned into pure gang rape. Pridot went from being cutely happy and enjoying herself to scared and struggling. That sudden shift from consensual to rape was just so dark and unnecessary, and the rape itself is just disturbing as fuck. It just really left me unsettled and pissed off, knowing that it could have stayed consensual and plenty of people still would have enjoyed it. But nope! It's hentai, so it's gotta be rape!
...Whoo. Alright, tangent over. Back to the discussion at hand. Continue to share your pet peeves (or major annoyances), or even share your thoughts on anything regarding this very post, if you so desire.