Well, I have nothing negative to say so far being a fellow programmer you've got many things right.
Design - I like simple, yet fresh design you implemented especially on the forums they don't even look like they did on the other sites. The colors are just right not to bland and not to flashy neither.
Speed/Optimization - This is where I was really impressed, the site loaded instantly on my computer where as to some other sites it would hang for a few minutes or even crash my browser.
Clean URLs - Props for having all pages now be mod_rewrite generated and not just on the forums like before, it looks cleaner and much easier to remember incase someone wants to share URLs.
Well, that's my feedback. Kudos I think all that hard work was worth it in the end and I haven't even found a single bug so far +1 for that too.
P.S The background changer is kickass!