Oh my, probably too many for anyone to bother reading. But let's start with isekais, be they of the reincarnation, transmigration or summoning variety:
1. The vast majority of isekais that revolve around the protagonist using their Wikipedia-brain to slowly introduce technology to this new world. More often than not it's in the form of food, and I haaaate that sub-plot in any isekai. Miso ain't that fucking good, my man! It also has some darker implications, in terms of how the culture of the other world is viewed as inferior, but I'll get back to that later. It can also revolve around other knowledge. Technology is a big one, and I think the only one that I have ever read that did it right was Release that Witch. And even that one gets to be a bit crazy with how much random stuff Roland can recall. The idea of using knowledge from a modern world is in itself not bad, it just is rarely handled well.
2. Slavery is totally okay when the MC does it. It's odd how these "culturally sophisticated" Nihonjin Messiahs don't mind slavery once they pop up in another world. I mean suuuure you won't mistreat your slaves, so I am sure they won't mind. At least they get to experience the utter SHOCK of getting to eat at the same table as their MASTER. Death March, despite me not really liking it, at least give a reason for the continued slavery and actually does resolve down the line if I am remembering correctly.
3. A leveling system of some kind where the numbers are not important. This could be a wuxia/xianxia/xuanhuan problem too, where I am fairly certain all of the ones I've read the MC has some ability that allows them to fight people way above their own strength where others would be crushed underneath the baddy's boots instantaneously, however at the very least there is some backstory reasoning for the "numbers" not meaning anything in those. It's perhaps more apparent in other tales where there are statuses and skills and all sorts of things. I don't mind LitRPGs, but if you do have a stat sheet it shouldn't be pointless. An even worse offense is when the stats are there to tell you how much the character has improved, rather than showing you.
4. This one definitely applies to other genres, as well. A conveyor-belt of cannon fodder baddies with no personality other than 1) Arrogant Young Master #323, 2) White Lotus #4112, or something along those lines. I do love me some face slapping, but it seems like all novels that rely on them will eventually devolve into a very repetitive story structure where you can just replace the name of the face slapping target(s), the reason for jealousy or ridicule and there you go, another 20 to 30 chapters that quite literally write themselves with a word replacement function.
5. All the women are "jade beauties", and they all seem to be having the worst day of their life (which usually means they are about to get raped) when the MC comes across them. But thankfully, the author allows the MC to thus display his power right in front of them and instantly make them fall for him despite more often than not being the coldest of cold jade beauty in the entire world whom no man has touched since they were born.
6. Gold to Yen conversion. It gives me all the info I need to know about what's to come in this novel and the quality to expect. It is an action that makes no economical sense. How on earth can you convert between currencies in a world that has magic and usually lags vastly behind technologically, has entirely different supplies and demands and our modern world?! And since money will never ever be a problem for the MC going forward, it's a meaningless paragraph that I still have no clue why any author would include other than to check off another point on the Isekai Trope list.
More generally, isekais just seem to revolve around the cultural superiority of whichever country the writer is from. It's not about exploring a new, vast world, but rather about "enlightening" these uncivil savages who do not even know about the glory of miso soup, soap or have thought far enough ahead to go dip themselves into the hot spring around the fucking corner from where the MC pops into existence That's why they tend to take place in fantasy/medieval settings, where the MC actually has inherent advantages, rather than in the future or in a sci-fi setting where they wouldn't.
For the more general novels:
1. The vast majority of male leads in CN "Romance" (if you equate Stockholm Syndrome to romance I suppose it applies). They are often selfish, cold-hearted, possessive and cruel womanizers. I get that part of the appeal is to then mold him into someone who loves only the female lead, but it's just very hard to get past the physical and mental abuse.
2. Somewhat connected to the last point, but applicable to all genres with a romantic subplot. Female leads who rely on the male lead. They're not independent, they don't have agency, they are just a glorified pet for the ML to raise. Even in stories where we are told that these women are the most powerful cultivator their age, or some knight that is an absolute badass, eventually they need saving and that's where the MC comes into the picture. All of their badassery is then wiped away and the only trait that remains is that they love the ML.
3. A lack of failure. I can look past a lot of plot armor that prevents the MC from literally dying, sure, that'd kinda give the story a rather abrupt ending after all. But when the MC is perfect, and that perfection extends to any companions (and let's face it, they're all usually women who are gaga for him) as well then I have no reason to continue reading as there is no risk of any harm ever befalling the MC, even if the harm is emotional from it affecting someone close to them. On the other hand of course are the novels where the MC lives a utterly tragic life with no joy, but depending on the writing and severity of the tragedy that is much more preferable than no failure at all.
4. An MC that does no harm.
5. An MC that does lots of harm. Unless it's some jade beauty of course, in which case the only "harm" will be in the form of the horizontal mambo
As far as the translations are concerned there are the more obvious problems like MTL'd content that fries my brain, badly edited translations if edited at all, etc. But the biggest one for me is adding translation notes all over the place. I don't mind a quick note when using a term that doesn't really have an English equivalent for the first time. But when the TLNs constantly comment on the story and try to get themselves over, at least have the common courtesy of using a footnote system and gather up all the jabbering that way.
I'm sure I could add even more to the list, but I've already rambled for far too long about this stuff. So let's just leave it at that for now.