Tomoyuki I enjoyed Trial Marriage for a long time, and then kept reading it for far to long. I think I made it into the 700's and it had been a LONG time since I had been getting annoyed with it.
What turns me off a novel are many of the things mentioned before, especially repetition and to many pointless sex scenes.
Sex is great and some authors have a real talent for writing the scene, but if it's not helping the story it is hindering it. I don't need to stand in the room every time the couple gets hot and bothered.
Pointless chapters in general. Much like Trial Marriage and several others I have picked up and dropped, when your story is told, end the book. Throwing in reskinned subplots and characters for 200 extra chapters in the middle of the primary story is infuriating.
As many already mentioned, toxic relationships that are 'all good'. I don't mind growth, I understand the redemption of a character. But I need to see it happen. Don't show me a guy that beats his girlfriend(boyfriend) or rapes her and then 200 chapters later because SHE(or he for BL) had some growth they are all good. I am ok with seeing a character do a terrible thing and realize it, sit with it, suffer from it, and crawl his/her way out of the grime of it. Then, and only then I might be cool with the idea that they can work through it and find a 'happy ending'
Harem without reason. I don't hate the harem idea entirely, I think it is entirely possible to have a healthy and loving relationship with multiple people. But most of them here are just a chance for the ML to get to have lots of sex with lots of different women. But with a reason, and proper development it can be an interesting read. Especially if he(or she) isn't a dude bro about it.
Advertising mid chapter. If an author tells me to check out there other book in the middle of the chapter instead of the author note section, I am closing that book and immediately removing from my library no matter how much I might like the story. You just kicked me out of your own story for a commercial about you, nah I paid for commercial free.
Lack of description or world building. You don't have to explain every blade of grass or go Nathaniel Hawthorne and his door on me, but at least give me an idea of where I am. And don't tell me what I am supposed to feel or see in little author notes throughout the story
EX: The buildings were tall, with roofs that curved inward. (like pagodas in china.) Pink flowers fell from the trees (think cherry blossoms)
EX: He slapped her. She was shocked. (grr I was so mad when he did this...)
There are more but I realize how long this had gotten, I apologize, though I am still posting it. :)