I remember how some forums used to show posts and replies in a threaded Usenet-style. Only the title was visible until clicked on and unread ones could be in bold.
I can't remember any recent forums letting me easily see which replies are new since the last time I viewed the page. I find it hard to believe people wouldn't like this feature. I'd like to see which are the new comments on multiply, slashdot, fark, and so on.