Like, the AU has all the worldbuilding done, there's action, adventure, and excitement... and some very familiar names doing very unfamiliar things.
Change the names and they'd neither be derivative nor out of character.
Plus without the fanfic connection more of them might get a character. The OCs are well drawn, the characterisation quick and clear, but the fanfic characters are pretty much a name, because hey, we know them already. Except we don't, because they wouldn't do that. So then they're just a muddle of no particular distinction. And the writer can clearly do better. So if it wasn't being fanfic, tada, sorted.
Also I always get hung up on how characters are translated into AUs. They retain what the writer considers their important distinctive features, but if I disagree, I just spend the fic rewriting it in my head so they're proper again.
And then there's that thing where they've had so much fun building in easter eggs, names we'll notice, power translations and so forth, they kind of don't give them much to do. Cameo overload.
Then there's genre features I can't be having with getting mixed with characters I otherwise like. Sliding timelines slid in inconsistent directions. Geography and religion renamed but not with any noticeable pattern. None of it stuff that matters, just stuff that trips up the reader, if and only if it's fanfic. Since fanfic relies on existing knowledge and AUs use it to do their own thing. If the story was just original, no tripping.
Of course if the story was just original I wouldn't have started reading it, but then I wouldn't have been disappointed by how my OTP were... not entirely present in the inspired by AU characters, and it explored none of the themes I like with them.
Fated love at first sight soulbonding fic seems very popular, but ... no. *shrugs*
And while I'm complaining about AUs: Steve Rogers in AUs is tiny pre serum Steve, unless you can come up with a very good excuse. Not just go lalala growth spurt, actually deal with his multiple disabilities.
I read an intriguingly persuasive thing that linked most of it back to an infectious disease listed in some props, one that's treatable or pretty much isn't around now, so by that logic it makes sense to have him grow up healthy. But healthy looks like Chris Evans, not like Captain America. There's a bit of a height difference either way. ...though I know movies are so inconsistent about heights it's easier to just ignore them. The height of boxes that must be involved to have everyone look much of a muchness in some shots...
I know people just want to keep their favourite version around, but the lack of logic bothers me.
And then there's the AUs where they just decide to erase age differences and hit rewind on certain older characters. Older being a very sliding concept that can happen to most anyone. But I don't get it why someone would be enough of a fan of a character to write about them, but only if they knock twenty years off their age. Not to explore backstory, or for an AU with specific age limits, just... because. That's their AU now, the young people version.
You just magic away all the little lines around their eyes when they smile. Why would you want to do that?
It's the AU thing of keeping the significant bits. You just get to notice repeatedly that what others value in your favourites is not the same as you.
Eh, it's 0451 and I haven't quite finished this thing yet, I'll stop rambling.