Let me be clear, he's no cormac mccarthy(my favorite prose is the passage from blood meridian describing the raiders). My bar for fan fiction prose is basically whether it annoys me, its a low bar. Most of what makes hpmor good is the cleverness with which yudkowsky subverts the original harry potter world to try to make a rationalist argument, whether that be by inverting a character or playing off the harry/voldemort relationship in thoughtful ways.

'"Voldemort," said the old wizard. "I understand him now at last. Because to believe that the world is truly like that, you must believe there is no justice in it, that it is woven of darkness at its core. I asked you why he became a monster, and you could give no reason. And if I could ask him, I suppose, his answer would be: Why not?

They stood there gazing into each other's eyes, the old wizard in his robes, and the young boy with the lightning-bolt scar on his forehead.

"Tell me, Harry," said the old wizard, "will you become a monster?"

"No," said the boy, an iron certainty in his voice.

"Why not?" said the old wizard.'

as you can see its nothing special, just good enough