It feels like I've seen lots of these chat clients on ShowHNs. Isn't there even a YC-backed one? TinyChat or something?
This one is nicely done though - here are my immediate thoughts on tweaks (mostly ideas from other in the chat rooms, but I've got a few mins so thought I'd summarize them here.)
* Move it to aggregate chatrooms by domain name
* Add a 'Always chat on this site' checkbox that pops it open whenever I visit HN.
* Remove the lobby
* Change "5 online" to say "5 people" or similar
* It needs history desperately. Just the last 10 comments would do as a start.