You'll notice that there's a new function: forum sections containing unread posts (in threads you've recently read, as this is a new function) will have a green icon instead of a grey one. Similarly, when viewing a forum section you'll notice that threads now have a grey or green dot, indicating whether there are unread comments in it (or the whole thread is new and unread for you).

We'll be making this a little more informative and intuitive over the next while, but it's the first step towards making the forum easier to interact with.