The lack of thought and worldbuilding that goes into the setting is one of my biggest beefs. To me, the setup of "A person from our world is in a strange other reality!" is the perfect excuse to go absolutely hog-wild with the world building because you can have characters explain literally everything to the protagonist who knows nothing at all about that world without it feeling clunky! You never need to have that issue where characters are like "AS YOU KNOW, we live in a constitutional monarchy ruled by the Mushroom King, who is the being with the most magic" to a character who lives in that kingdom and ought to know that.
The perfect opportunity to have a world with strange customs, strange gods, strange food, strange terrain with the chance to seamlessly explain all of it to the protagonist and what do they make? Oh, it's basically a less imaginative version of a generic JRPG or something. Like it's not even as inventive as the popular video games in the genre it's inspired by; Final Fantasy XIV has some pretty interesting world building in terms of technology, Aesthetic, religion, creatures and cultures, but isekai stories it's always like "there are elves, their culture is they live in forests, I guess.". Hell, Final Fantasy X practically WAS an Isekai and it used that to give the setting a really rich culture and history that made it truly feel like exploring another world.
Escaflowne has already been mentioned twice on this thread but I'll give it some more praise because oh my god, Escaflowne really did have such a wonderfully unique, alien feeling world. That's Isekai done right.