The design of UX.SE predates the "flat-design" craze that took most of the design world by storm last year.
- white flat monocolor logo (the words User Experience are part of the logo instead of being text so they won't scale well with the page zoom)
Most likely it's locked into an image because in 2011, web fonts were still a new-ish web technology and the team here tried to keep it simple with site designs. Only in our most recent site designs (Workplace.SE and GraphicDesign.SE) have we started to use some web fonts. The design most likely called for a specific font (looks like Helvetica Neue Light) and we had to use an image.
As far as your other comments, I'll just quote this from Jin's meta post:
[Dmitry and I] both agreed to go with a clean and simple design. It should serve as a "pleasant shell" to the excellent content we have. It shouldn't be over-styled to overwhelm the content, but at the same time it needs to be polished enough not to be barren.