Since I visit here every day I get to know the styles and personalities of other frequent posters, I have seen many comments by them, and seen a lot of their blogs.
This adds to the conversation, since I can often tell why usernames I recognize argue in a certain way.
I also often remember previous discussions I've had with certain users. This is a good thing, since I know the intellectual integrity of a user I recognize when a back-and-forth dicussion ensues.
The usernames on this site are very small and inconspicuous. I take that as a sign that HN wants us to focus on the content of the message rather than the author. When I'm on sites which place more visual emphasis on the author (names in big fonts, avatars, etc), then I always notice who writes comments.