I think we just need to teach people how to use the tags. Maybe I could make the interface more clear? I just need to learn to program better.
One problem with adding more forums is that people put things in the wrong forums all the time... so that creates more work in moving things around (even this thread was originally in The Lounge). Another problem is threads are about more than one thing. Let's say there were a forum for build threads (projects), and also a forum for choppers, one for bobbers, and one for trackers, where would a build thread about a tracker go? The way things are setup, you can put that thread into both categories. It's not perfect but neither is creating tons of specific forums. I know you are only asking for one, but that might make things more confusing if most things were posted in the garage using tags to categorize them, and one or two things had subforums.
I like being able to put things in more than one category and the best I could come up with for now is using the tagging system to do it.
By the way, anyone with more than 3 posts can add and remove tags (even on other peoples' threads) and help keep things cleaned up on the site. Think of a thread's tags like the categories it belongs in. In my opinion, it should only have 1 or 2 of the main tags used in The Garage (I call them primary tags), and the rest (I call secondary tags) should be more specific about what the thread is about.
Have you read this?
The Garage Forum Topics and Categories (Tags). There is some great information in there as well as in that whole section:
Site How-To Articles
I'll try to go through and clean some of the threads up that shouldn't have the build tag and see if that helps.