A LOT of forums try to help with a huge volume of users by having an entire hierarchy of forums. The problem with this is that things SPAN categories. I’m thinking of HEAVILY trafficked forums like XDA Developers. They have sections for each phone and then even sections for the vendor-specific model. Seems to me if you wanted to be a successful forum replacement, you’d have to target the most successful forums (e.g. XDA Developers) and review their actual usage of features and determine how to solve those same problems they are solving, but in a better way.
This is where I think allowing multiple categories (a la SO tags) would be a better solution to the extensive hierarchy of sub forums which a lot of big popular forums tend to use to help users find things and track things.