I hope it’s okay for me to add my thoughts to this!
For a while now I have observed that a lot of people, even people who read a lot, struggle to talk about what they read on anything except the content level. This includes students in my undergraduate lit studies classes! They can tell me what happens in a story and maybe how it made them feel, but if I ask them why they feel a certain way, they can’t point to it. It blows their minds when I go through a couple of examples of how words make you feel things (like the difference between “red drops” and “crimson spatters”). They are not paying attention to conventions or formats or narrative techniques and they struggle to comprehend the function of those things.
The extension of this is that most people don’t know what “good writing” is. They just don’t. This has always been the case to some degree because most people only care about whether or not they like a book they’re reading, although of course “good writing” usually makes a book more enjoyable to read. But it also appears that a lot of people “read” without really taking in or paying attention to the actual words on the page beyond what they need to imagine the scenes. This is why some of the most popular/successful published writers out there have incredibly dull prose: people prefer straightforward writing that doesn’t distract or confuse you with complexity of language.
The unfortunate consequence of the increasing literacy crisis is that the standards for what is considered “enjoyable” keeps dropping. Readers have lower standards for what they will rate 5 stars on Goodreads and that’s all publishers care about, so they can get away with putting less effort into their output, so the overall quality keeps dropping too.
There’s this idea in literary studies called “the affect” which bluntly put just refers to the physical and emotional reaction like when you watch a scary movie and your body reacts as if you really are being hunted by an axe murderer. The affect is a huge factor for why most people engage with media: they want to feel something and the more intense the feeling, the better. The thing is that the human mind and body are not that complicated and the buttons to make you feel Sad or Happy or Horny are very big and easily pushed, which is why there are cute puppies in commercials and why some writers try to make every single sentence some kind of climax. Increasingly, people appear to mean “it made me feel things” when they say “it’s so well-written.”
So the best way to make people respond positively to your writing is to make them Feel Things and you don’t need to be subtle or clever about it. In fact, it’s better if you aren’t. If you can make them Feel Things strongly and also repeatedly, even better. In a fanfiction context, that means readers as a demographic prefer frequent updates with a high density of humour, sex, or intense emotional scenes and they don’t really know whether it’s well-written or not.
And fanfiction doesn’t need to be “well-written”, that’s not the point. We were all shitty writers once and there’s no test you have to pass to write fic. I do think there have been some shifts in how people engage with fanfiction as readers that exacerbate individual works getting so much attention as well. My personal takeaway from the Claude-fragment situation is less “wow, people have shit taste” (because this has always been true, the art that appeals to the most people cannot be the most complex or nuanced because then fewer people would like it) and more “this is a direct result of the lack of reading skills and push towards instant gratification we have been observing for years because people are losing the ability to be discerning about what they enjoy”