I think it would be nice to be able to get a notification for when people comment on a particular post or comment. Makes it easier to keep up to date with discussions. For posts, if it's not too hard to code or too server heavy, there would be an option to choose between notifications for only top level comments and all comments. Otherwise being notified only for top level comments would be fine imo. For comment chains, I imagine it would work by subscribing to one particular comment and then you get a notification everytime the chain of comments grows from that comment on. That way if you're interested in the full discussion, it's easier to keep track.