I know these are obvious pet peeves, but I have three (two about endings, one about characters)
Cliffhanger endings. I want to feel like i’ve finished the story. One novel ended at a cliffhanger, and it’s only saving grace was that there was a sequel. Still, it felt like the author didn’t want to write a long novel, so he/she (I forget who the author was) just split the story right down the middle, which annoyed me.
Don’t make an ending that makes the story pointless. I’ve read one novel where the characters realized that they can send a letter to their past selves to give them a chance to try again when they failed, making the entire story seem pointless.
No whiny characters. Characters don’t have to be perfect, and they can get angry. But they shouldn’t complain about everything that goes wrong.