Basically, the 'What's New' does the same as the old 'New Posts' thing. ALL the posts shows are ones you haven't visited at all, or ones that have replies since you last visited (or replied) to it.
If you visit out lesser busy forums, and you read all the posts shown in the 'What's New' page, then hit 'What's New' again, it'll say "Sorry there are no new posts to view, click here to search for all posts made within the last 24 hours". And the link is basically the 'Today's Posts' button, so it shows all posts within the last 24 hours. So it's only shown if there are no new posts for you to read. Quite clever really IMO.
So it's the same.