Probably spelling mistakes and odd grammer, I get that English might not be their first language and/or they proofread it themselves but didn't notice the typos, I can excuse that, I deal with it all the time with translated works, but sometimes it's to a point where I can barely understand what's happening, so it just makes me less invested and immersed

The one big paragraph thing as well, literally no breathing room between moments and easy to get lost in

I'm pretty sure there's more but I'm blanking right now

THE LACK OF OR WRONG USE PUNCTUATION MARKS !!!! it’s understandable that majority of writers esp online that they don’t know how to properly use it but i think i makes the writing kinda difficult to comprehend because they’re kinda just going it a straight line. for example, i can’t tell the difference between ill and i’ll because the apostrophe is missing ┐⁠(⁠ ⁠˘⁠_⁠˘⁠)⁠┌

The big ol' paragraph, 10y.o roleplay (she smiles, -she smiles- constant authors notes every paragraph), messy spelling and grammar with no warning of so, etc. it's hard to choose one.

Wattpad ruined first person for me. That's all I'm saying or when I person just doesn't use common sense to write a book

When they write dialogue like this... -Erm what the sigma?- said Jonathan and he ate all the cheddar cheese -DANG YOU JONATHAN THE CHEESEATER-!!!! screamed the villagers.

i dont read much fanfiction anymore but i hate when theres grammar mistakes it distracts me from the story IT KILLS ME!!!!!!!!

i have a Bunch .. this is more like. general fanfiction things that upset me rather than things about the fics themselves but

1. request books with no fics in them. i understand that maybe people just don't have the time or energy, but they always clog up the feed. if you don't have the motivation or time to complete anything, then just.. Don't take requests... 1.5 when people post fics with nothing in them to "save their tags". once again you are clogging up the feed 😭😭.. drafts are there for a reason..
2. like op said, when the entire fic is just one big paragraph. it's really difficult to focus on a giant block of text, especially if you're reading on a longer screen.
3. when the author gets bored of writing the fic & decides to end it in the least satisfying way humanly possible. i read a fic once where the author didn't want to write the pairing anymore & decided to end the fic by having one of the characters be killed by a falling meteor.????? it still baffles me to this day like you could have just marked it as complete 😭
4. seeing the author's poorly disguised fetish. Out of nowhere. In a safe for work fic. need i say more
5. poor prose.. just generally speaking.. i don't mind lowercase fics & i can sometimes even overlook poor grammar, but if the flow & rhythm of the writing is wonky then i'm probably not going to read it. there's more things i don't like but i forgop 💚