Currently, the easiest way to track if somebody has replied to a comment of yours or recommended your comment is to check your profile. Whilst this works, I'm pretty sure an easier way would be welcomed by almost everybody.
I therefore propose some form of notification system tied in with the user account dropdown in the header. When hovering over your avatar, a list of recent notifications ("x recommended your comment y", "x replied to your comment on z" etc) would be displayed with a link to go to a full list of notifications ageing back to your last visit. Think how Facebook style notifications work and you'll get the picture.
I really do think this could be beneficial. It would certainly be welcomed by myself.