Well I don't want to be negative but... there are some things I do not like.
This is a nice design, but in my humble opinion is not a great user interface.
Eyeball soup. The font sizing, coloring and serifs overload make your eyes wander all over the question pages. There is no emphasis or structure to guide you from one question to another visually. It also very hard to skim questions. This lack of visual distinction also makes reading meta data difficult. I think serif was a bad decision, I can tell due to the difficulty of skimming question I will spent much less time on this site.
The header is to large, this is a Q&A site , not a blog.
One important reason stackoverflow ( and sites like reddit, craiglist, etc) are very popular, they understand the difference between user interface and design. Remember this is a Q&A site modeled after stackoverflow where the data is the design!
ps. The search is way to small.