FanFiction

Oh gosh, there's not one specific thing. Really, if I decide I'm just not enjoying a story, or not enjoying it anymore...I stop reading it. That doesn't mean there is anything objectively wrong with the story. It's possible that my fixation has shifted, or that something arose that I just didn't care for in terms of how they presented a character or situation and it struck in just such a way.

This actually happens more often with novels than it does with fanfiction, as fanfiction authors tend to be fairly forthcoming and they use a lot of tags.

As an example, I remember getting a few chapters into a novel once that portrayed (not in a way that at all endorsed) a very abusive relationship and did so in such a heart-breakingly realistic way that I couldn't get past it.

The story itself was an absurd, fantastical, bizarre work of fiction that just so happened to portray certain very real dynamics in a way that felt jarring. If anything, that is to the author's credit, but I noped out not terribly far into it.

Sometimes I just get bored with a fic. Usually if it takes too long to update, and by the time the next chapter rolls around I've lost interest in the fandom and/or forgotten everything that happened in the fic. I generally don't like slow pacing either, so if I'm itching for something interesting to happen and the story kinda stalls for a while, I'll probably drop it. (Though if it's completed, I can skip ahead through the "filler" if I'm generally into the story otherwise)

Or it had an interesting premise/tags/summary but doesn't really deliver. Either the characters veer into OOC, or worldbuilding/magic/fantasy things integral to the story aren't explained so the reader understands what's going on and why. Though I guess those are more "bad writing/storytelling reasons."

Outside of that, untagged tropes/squicks. For some of them, a brief mention is fine, but generally if the story starts heading in that direction or focuses mostly on those things, I'm out.

Massive OOC turns for established characters, or sudden tone shifts that don't make sense in the context of the story. I don't often mind OOC in the context of the source material but if it's OOC in relation to the rest of the story, that's a hard nope from me.
A sudden disappearance of any & all smut in an M- or E-rated story once a relationship is established. Gah!
Likewise, when pregnancy/children are portrayed as the pinnacle of a romantic relationship. I'm fine with babies/kids being part of the story but when they're suddenly pushed as the focus, my tokophobia & childfreedom start showing.
Apart from those, it's mostly ongoing obvious SPAG issues.
And sometimes poor tagging, although this is usually the case when I'm already finding that I don't like the story. If I've already been happily engaged with it, I don't really care so much about the tags by that point.

So somethings I'm going to list I feel are outright bad writing, but considering how popular such stories are I'm going to list it anyways because it's clearly not a universal opinion. (To further note, these are things I apply to both original and fanfic.)

Green Aesop styled stories that treat humans as if they aren't animals and don't also come from nature. That they are all inherently evil. It completely disregards how most damage isn't from majority of humanity but billionaires. It's also massively ablest. If it doesn't disregard, it outright states that if you don't have a body that that handle being in the wild, fuck you.

That is, unless the work has decided to go, "nature is innocent" route. How to instantly tell me you have never worked with animals, done anything in nature, or know anyone who has. Yes, there are wild animals that will help those outside their species. But don't ignore how there are there are animals that will kill not to eat- but just for fun. Some of those animals are the same species as the ones who will seek to help.

On that theme, but I don't think aren't bad writing in general. Just massively poorly categorised by people who like staying genre-blind (and this complaint is largely aimed at fan works and specifically a certain fandom):

Works that take the high-fantasy setting, claim its going to deconstruct and show how certain things would be applied realistically... but what they actually mean is they want to write grimdark with no research. All the wild animals basically act the same regardless of their inspired background. Turned into common house cats or sport dog breeds. That is, if they aren't just humans in fursuits.

Wanting to tell stories like this isn't bad, just, dammit, be honest about what you actually want to write? This misinformation is so bad that in this same fandom people often say my work is grimdark.

One I admit isn't bad writing and is a pure pet peeve: Characters who know who caused them harm but lash out and deliberately hurt others who were not involved. This is something that happens all the time in reality. But I really, really cannot stand it in fiction. (There is only one story exception I have but to explain will make this post extra long and it's already 9+ paragraphs hhhhh)

One similar vibe: "no one understands me!" or, "you don't get it" by a character who never tried to explain what was going on. Extra points when this is yelled at someone who is completely new to the situation. Like? No shit they don't know? Why are you talking as if they have been in this situation as long as you have?

Stories that are more deeply invested in describing the look of every room and outfit instead of moving the plot forward. I'd might be able to forgive it if it was a Locked-Room Mystery or similar, but that has not been my luck. A big warning to me is when authors phrase celebrations entirely around their wordcount. Instead of what exactly that wordcount accomplished.