After reading an interesting discussion's comments, I'll often close the window, do something else, and return to the discussion to read the new comments.

However, it is annoying to re-read the same comments I've already read. Sometimes I'll refresh a discussion 5+ times to see the new comments, and it's harder and harder to find where the new comments are located. I suggest that comments posted < 30 minutes before be marked somehow as "new", either through CSS or some visible indicator.