I dislike isekais where side characters madly fall in love with MC without any in-world reason. We as readers know that the fall for MC to play into the wish fulfilment, but at least give the characters some motivation other than “just do it so the reader feels good”.

Another point would be that most every isekai world somehow follows the same Japanese societal norms. Like using the first name is a big thing or the isekai language has distinct casual and polite variations. It’s just boring if the world building is just Japan with a romanticised medieval Europe paint job. Throw in a dab of slavery so MC can get a harem of girls who cannot disobey him (and he has some easy targets to kill and feel morally superior about in form of the slave traders - but that’s secondary to his own slave harem), and you get the basic setting for 90% of trash-tier isekai.

I’ll start. When it comes to protagonists:

I’m tired of bland, “average person” protagonists. I get that they’re a bit more relatable, and it would be OK if they weren’t every freaking where, but at this rate they make me roll my eyes and say “oh great, yet another of those…” (Example: Tadashi, from Alcafus. I’ve seen rocks with more personality than him.)

I hate meaningless big numbers. Who cares if your protag’s power level is 1 billion or 1 trillion, if the average human is 10? At some point the number loses any meaning. (Example: Isekai Cheat Magician.)

On the other hand, I love when the protag has some issue that makes you say “dude… that’s fucked up”. They might be less relatable, but this makes them look a bit more people and less like a shell written “insert yourself here”. (Example: Rudeus, from Mushoku Tensei. The guy is a disgusting creepo - and that makes him interesting.)

I love Uncle from Another World (Isekai Ojisan) because it does isekai satire so well. I’m seriously laughing out loud every episode. Watching it slow so I can savor it

It’s basically what I thought Eminence in Shadow would be. EiS is self aware and pokes fun at isekai, but taking a step back… it’s still a standard isekai. To be fair I haven’t finished S1 of EiS yet so I may be too quick to judge. But to me it feels like they stuck in as many isekai tropes as they could, and an edgy sarcastic MC, and called it satire… I mean it kind of is, but idk

So to answer the question: basically the overpowered MC, the slave harem, the weak attempt at blending in so nobody can guess MC’s real identity as a god-tier warrior. How MC outsmarts all the natural denizens of the isekai and instantly understands how their world works and how to game it.

I also dislike when MCs basically completely forget their old world, they never stop to reminisce. I get that that’s kind of the point of isekai, escapism and not thinking about real life… but it’s unnatural to me. They bring their memories with them but throw them away.

Of course some of the better isekai do talk about both worlds. Like Re:Zero comes to mind, and ofc the ones where the isekai is a literal video game, and they travel back and forth.