A few times this has been suggested, by myself in fact. It would do many things such as filter out already read threads in forums, and subforums under that.
In addition it might solve the problem of people getting posting in the wrong subforums in an ongoing basis, because people could more easily see what is happening in the sub-forums. It might be better as an addon, not sure.
If I can find, or anyone else can find, the other threads about this I will merge.