Oops; your reply happened between the time I loaded the page and the time I hit the reply button. I think it would be cool if Reddit-style comment systems could show new comments in realtime, Wave-style. The number one complaint about wave is that "it's cool, but I don't know what I would use it for". People already know what to use sites like HN for.

I'm not sure how usable it would be to have comments suddenly appearing -- it might get overwhelming if there are over 1000 comments, as on some Reddit articles -- but I think it could be pleasant on a smaller site like HN.