Hi Andrew,
I agree with and can totally identify with this issue - so think it's a great idea! my only concern would be that with all this flexibility users may get confused as to what forums or threads they are subscribed to. So be missing out on posts that they feel they should be getting and vice versa.
So perhaps along with the capability we would need to design an easy to use centralised interface for each user that displayed their forums (or groups of forums) along with their subscription settings and then ideally allow them to manage them from there.
Regards
Mark