When plot and character development make way for harem.
I just don't like harem. Even a love triangle is sometimes already too much for me, especially the "we fight (meaning fuck) fair and (almost always) with everyone involved" kind which ist just Mc's harem but with just two members. Can't tell me the relationship is poly without showing how things go without MC involved...
Or when new characters don't get any kind of development or growth after their "arc", which almost always is just
Introduction of new character -> Problem -> MC solves Problem -> New character is now friend/haremon/plot device -> That's all, folks
Forced School arcs or forced tournament arcs are also a no-go. Those almost always appear and only serve to show how AWWSUM MC, YU DA BESTT the main character is. Always the same underdog story as well, Yes, they're easy to write, and yes, they are fairly hard to mess up, but that's it. No risk & no return for a WHOLE ARC is too much. At least do it the way TsukiMichi did and focus on other characters or introduce new characters during those arcs...
And last but not least: Bad editing and/or grammar. That's my personal Vietnam. If you're at least a bit proud of your story then please, please, PLEASE get yoursef an editor. And if you think your story sucks get yourself an editor. Or at least force it through grammarly or word...