MCs who are down to kill every living thing in a country because some random dickhead bumped into him or something. Like, we get it, OP is so strong and such a badass, he doesn't have to commit genocide every other week to show it off.
Actually, any trope that takes the super strong MC we're supposed to root for and turns them into horny murder machines who only know how to kill and rape anything in their way. It often just comes across as lazy author venting and makes me almost immediately give up on the story. Not that every protagonist has to be a pacifistic pushover who lets people die for no reason, but by the time our super strong MC can do things like exterminate an entire clan or something, they should totally be able to just set their loved ones up somewhere safe and have the maturity to ignore provocations from weak mobs.
Unrelated, but I also hate when powers ramp up too fast and the author writes themselves into a narrative corner where the only way to create conflict is to power creep up to infinity. By the time the MC is fist fighting ancient deities or whatever, you should probably either start wrapping things up or prepare to transition to MC enjoying their life peacefully after all the adventuring is done.
Lastly, I hate when the narrative just forgets about girls in the MCs harem or just abandons them along the way. Obviously, something something realism and not all relationships lead to romance, but come on! You spent fifty chapters on this girl and how obviously in love with the MC she is just to ditch her and forget she exists for five hundred chapters? Why not just skip her being in love with him if you were just going to unperson her anyway?