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.
So why doesn't this exist? It doesn't have to add work to the server. The server doesn't have to keep track of when a page was asked for. The client can do that, say for the last seven days (changeable), and bold the title of new posts.