The premise of the site is good, but I think some of the forums are unnecessary and a bit unclear. For example - Cabinet Design - if it's about people designing and building cabinets, it's "In Progress Builds", if it's a tutorial on what to think about, it's already covered in "Amp Building Tutorials", and if it's related to the cabinet design of production/already made amps, it's covered in one of the first 4 forums.
Could do with a general chat forum up the top?
Guitar Setups - if it's for electric it could go into 'electric guitar', if for acoustic, similar. Just feels like it's un-needed with those two already there.
And as for songwriting - you only really need one forum to discuss all aspects of songwriting, same for homerecording.
All the really successful forums I've ever seen develop small 'sub-communities', (take Harmony Central's Effects forum or Amps forum for example), of members that post on the board most suited to their interest.
If you have too many categories, people thin out amongst those other forums, and those sub-communities don't develop, which keeps the board as a whole thin at a certain number of members.
I think the forum's a great idea, just needs a bit of refining to make it clearer - so people know exactly where to post whatever they have, you know?