It would be useful to be able to subscribe to a forum sub directory as well as a thread. You would then get notified if a new thread was posted in that directory. The format of moving threads to the top of the list when they have a new post is good. However, it does have the side effect that a thread you may have enjoyed may be quickly pushed down the list by threads with a lot of posts and you may not notice it once it slips onto older pages. Maybe this type of subscription is already possible and I just don't know about it?
Use this... http://forums.digitalpoint.com/subscription.php?do=addsubscription&f=65. Check Thread Tools.
While you are in a forum (the screen that shows a list of threads), go to the "Forum Tools" menu. That lets you subscribe to the forum. You can also subscribe to an individual thread from the "Thread Tools" menu as WhatiFind mentioned.