Just some stuff that I come across often:
Would save me a second of time and effort if the "Username" field would get auto focused on the login page. I know it's a minor improvement, but I like it ^^. Facebook does it, for example - I like the way I can just start slamming on my keyboard as soon as the site is loaded, without having to click the "email" field.
The header gets a "fixed" position as soon as I scroll down anywhere. I find that useful, but in my opinion, something is missing in the fixed header when I'm reading a long post. That is because I often want to go navigate to a specific forum section from that page out (is this correct English?); using the back button and refreshing that page is more work than clicking on the section I want to navigate to and go there straight away. Why not include those section options in the header that are also there when I scroll inside a section's posts overview? :)