I think something along the lines of the Digg Spy facility - http://digg.com/spy
- would be quite useful.
The new forum layout is pretty handy as you can jump from one forum to another pretty easy, but it doesn't cater for the people (like me) that sat on the forum index and refreshed every now and then to see which forum had new activity.
I know the AJAX 'spy' applications take up resources as it has to query the server ever 'x' number of seconds, but i suppose you would need to weigh up the difference between people constantly refreshing and jumping from one forum to another, against the number of queries required every 'x' seconds.
Anyway, i guess it is soemthing for the wish list when you have go everyhting else done.