The orange-white design makes it look much more modern, and clean. At them moment it's almost only a design change, but new features will be coming soon, as promoted.
The new appearance is for writing, the dashboard and the UI the writer sees. No design changes were made on the actual blogs. It feels the right way to the rebranding to Google Blogs. 3rd Party plugins like Zemanta, do have some problems and breaks the new UI. So we'll have to wait for them to update their stuff.