We have an issue where users leave comments on Diffs and the people that should respond to those comments don't get notified because the state of the diff hasn't actually changed (we look at the Active Revisions panel a lot). We then end up sending messages like "did you get a chance to look at my comment on this diff 2 days ago?" This happens more often than not and everyone forgets that they can mark the diff for further review.
I can imagine either another section in the Active Revisions panel titled "new activity" or something, or a Herald hook to take actions on comments.