I think its a nice idea to let users favorite the categories and tags in starting (means just after registration), as it is done in Quora. And if a good notification system is bundled with it then this would raise Q2A level to sky high.
But when you say about jQuery Drag and Drop idea, then that is on how much interactive you want to make it. A simple favorite button beneath every category would also do the task (as we follow users and some icons when we register to twitter).
Secondly, I liked the checkout jQuery concept. It is interactive adds more fun to working of q2a.
All this force me to think, can we add a pic to every category (like we can add pics to tags using a plugin). This would help in better telling and expressing the category to new users.