I have a... few. The first, and most impactful is characterization. If the character doesn't FEEL or read like the character they're supposed to, it ruins the immersion. (Unless you are actively TRYING to create an AU of some type).

More so an observation than anything as an English Degree holder, is of course sentence structure/flow; basic formatting. Yes, there are MANY exceptions to the rule and I usually overlook them. It's not really a peeve though, mostly just curious observation.

Using slurs. I don't usually like reading "c*nt, tits, etc." in writing. I understand why it's there though, and that's totally fine! It's just something I tend to look past, especially when the story is very good :3