HN feels quite usable to me. The only problem is the few remaining links based on closures (x?fnid=...), because they expire and it's just dumb. I may be mistaken, but I think closure-based links have been slowly phased out over the lifetime of HN, and now they seem to be used only for "more" and "logout". Why not throw them away completely?

About possible enhancements: ajaxifying the "reply" link, as billswift suggested, would help. Ajaxifying the "edit" and "delete" links would definitely help. LW does both and it's much more convenient. Making the vote buttons light up on hover would make the interface feel more consistent, because links already react to hover.

One thing HN does very right (compared to Reddit and LW) is that users' "comments" pages are threaded, not flat.