Added a little bit of code, and am eager to see whether it works. In short, for now on, when you return to a comment thread, all comments that have been added since your last visit should be background-highlighted – I’ve chosen a very light green background for now.
I think this feature would be a serious time- and eyeball-saver at blogs with long, lively, and complicated comment threads – but only, of course, if it works well and predictably. I’ll test it out for a while, and, if it does work, I may add a legend somewhere at the top of comment threads to inform/remind users. Anyway, let me know if it works for you, or doesn’t.