For me, the two green horizontal bars: Login and whatever is next to it are on different levels. Seems odd, I am using Chrome. Also, that's a long list of topics. Have you thought about collapsing them into general topics for the main page.
for me i think you should limit the categories first because most users that see lots of categories that have 0 posts wont register and use the forum. make a more general category then just slowly break it down when they already have lots of posts
Good Design, little changes necessary for the top banner - logo - and you can add 6 adsense codes in a page.(3link units + 3 ad units) So integrate it in to the page.
I would take out the forum one level down and provide more info on site in the main page. design is average, but altogether clean