I would be hard pressed to find a phone that had a better overall design. I'm not a huge fan of Samsung's plastic phones, but this one works. The Focus S seems impossibly thin and light. The first thing people notice when they hold the phone is how thin and light the phone is, especially with a large 4.3 inch screen. The plastic build is the 2nd thing they notice.
I've always been a fan of the WP7 OS, and I think that on the surface it is my favorite mobile OS. Sadly for WP7 it really only has a surface, due to the lack of an App ecosystem. During my review period of the Focus S I constantly felt that the phone was held back by the OS.
Besides the app ecosystem there really isn't a single thing I would change about this phone. Performance is great, the phone looks amazing, and it has a stunning Super AMOLED plus display.