We recently implemented a commenting system on BetaList. It's still under heavy development, but here's an overview of the gems we're using so far:
acts_as_commentable_with_threading
provides most of functionality you needacts_as_votable
makes the comments votable (so the best ones can rise to the top)mention_system
handles @mentions. this is a very young gem, but seems to work finemailboxer
is a very robust notification system (we notify the startup when there's a comment, when someone replies to your comment, and when you are @mentioned)