Well, from a design standpoint I can admire the fact that it is very clean and well executed. Personally, though, I found it somewhat depressing.
If I was a forum member, I might be driven away from spending large amounts of time there due to the plain, monochrome sceme of your forums. My advice would be to at least add some red buttons or borders. You might want to check out
http://www.clan-316.com/ubbcgi/ultimatebb.cgi - they are one of my favorite forums, and they use a mostly gray color scheme without making it look dull.
