I do not have an absolute checklist, my rating can be influenced by my personal enjoyment, accuracy of tags might take a star down (outright missing tags/content warnings, or tags with nothing to show for them for a long time) and I also noticed I might be more generous with short stories, but the following is the general guideline:
1 - Literary trash level.
2 - If your story has at least something going for it, even if it is generally bad, it will not get a 1-star.
3 is for mediocre books, not great, not terrible. The grammar is poor, the writing style might be awkward, but it is readable and enjoyable. Or the story might leave things to be desired, but makes up for it in good grammar and flows nicely without too obvious issues. Or a really good story, but with major flaw or flaws. You get the idea.
4 are great stories with only some minor flaws. Good grammar is a must.
5 - Story, flow, characters, style... this book has it all, and very few issues to point out, if any.
Most of the books I do rate end up either 3 or 4. A plenty of the books I check out, however, I do not rate at all, because I'm undecided or they're not what I'm looking for.
A 1-star, I might have given two times over all the stories I have read over the years, on both SH an NU combined. One was hard wish fulfillment, no decent comedy, no suspsense, not even slice of life, inconsistent to the next level with incredibly dense MC and the characters could not be more flat if you tried. The other, I don't even know what it was about, as I only barely recognized the language as an attempt at English.
Unfortunately, it feels as if most people rate a story as either 5-star or nothing at all, and so you get a practical result of ratings where 4.5-5 is good to great, 4-4.5 is anything from "somewhat alright" to good, and basically everything under 4 starts is generally bad - as long as there is a decent number of ratings to begin with.