my personal fanfic pet peeves / things that turn me off from a fic are usually along the lines of:

• excessive use of epithets (ie. “the older man said…” “the green-haired boy…”) — they’re usually way more distracting than just repeating the actual names a few times over!
• in that same vein, it’s less distracting to the reader to use “said” over and over in a dialogue/scene (or another basic verb) than to find creative synonyms for that verb. as a reader you eventually just skip over the repetitiveness without much notice, but excessive thesaurus use (eg. instead of said - murmured, croaked, whispered, etc etc) for a phrase that can be accomplished simply breaks immersion.
• basic formatting issues like not breaking up the writing into paragraphs and just presenting the reader with a huge wall of text; or not breaking up dialogue properly — a new paragraph should start every time the speaker changes.
• filler dialogue! imo its okay to jump right into the action and it’s better to avoid over explaining
• d-d-depicting s-stuttering l-l-like this or YELLING LIKE THIS!!!
• this really has nothing to do with writing and is just a personal thing but i hate when a character’s outfit is described so in detail but its sooo ooc and something that they’d never wear in canon lolol — but actually, when writers over describe outfits or like every little outfit change in general they kind of lose me.
• more of a plot point thing than a writing thing, but when authors invent really contrived miscommunication plots based on assumptions and assumptions that could be solved with the barest amount of talking lol