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.