More important, I think, than displaying the number of new comments is making it possible to /find/ them. The reordering of comments is usually a great thing, but in a relatively involved discussion, it can become quite a chore to find that new comment.
I'm not sure what the best way to implement it would be, from either an algorithmic or HCI standpoint, but it certainly would be nice if you could come up with a way to make new comments stand out in threads. (Preferably with new defined by when the user last viewed the page, rather than being a static global definition.)
