I'm a big fan of the navigation drop downs, although I would modify the header a bit more to make it cleaner. Also, when you scroll and the navigation sticks, that should extend 100%. I like the unique forum icons, that's always the way to go, makes it more user friendly.
I see that you have the space up top to sort latest threads based on category. You seem to have a lot of decent content. With this much content, why don't you have a homepage that showcases all of it in a better way as oppose to limiting yourself to tabs in a 3 inch area on your forum page itself?
Examples:
http://chefclub.net/
http://www.digitalweddingforum.com/
http://www.pearl-guide.com/
http://www.quilledcreations.com/