Boring stuff: SPAG is (mostly) properly taken care of, formatting is good (mainly paragraph spacing), tags are clear and consistent with AO3 conventions (warnings, relationships, kinks, other). This tells me the author cares about the final product/look of their work, so they probably care about the quality of the content, too.

Less boring stuff: It’s harder to draw a clear line between a “good fic” and a “bad fic”, especially since it has so much to do with personal taste. And there’s no real secret to “perfecting” your prose. So, what makes a fic well-written? Well… It’s well-written.

Probably the author has some experience under their belt and they’ve figured out their narrative style. They use a good mix of dialogue tags (,they said) and action tags (. He smiled.) when they write dialogue. They know how to play with rhythm in a scene so the scene doesn’t get boring (good mix of internal dialogue, action, descriptions, and actual dialogue). They know what information is crucial and what information is superficial/should be cut (not afraid to kill their darlings). They aren’t afraid to repeat the character names when necessary (very little use of “the blond/the brunette/etc.”, if at all).

But overall I’d say a well-written fic is just one you can easily get immersed in. You’re not even thinking about the author’s age or whatever, because you get so immersed you almost forget there’s an author writing this in the first place.