Actually this is the first thing I missed in Q2A. I get the idea that it is a tag based technology for quick consult, but the subcategories would allow us to organize the database and create a kind of "library" to permit the user to navigate through it instead of just searching for a tag. This is nice when you don't have a question to consult but just want to learn something new. for example.. I click in the first Category "PHP", than the sub-category "Framework", than the sub-sub "Zend" than sub-sub-sub "Technics" than I get to a nice QA that was discussed a long time ago that I would never think about searching it using any tag ... It is a silly example but you get the idea.
By the way Q2A is great!