God-Emperor
Deity
You're all missing the central point (IMO), that if you switch to the desktop in Win8 what you get is just an improved version of the Win 7 desktop (less start menu, but with a more generic search that actually works at least as well once you get used to it).
No metro.
No tiles.
Just a normal desktop.
The Metro UI is for touch screen (primarily tablet/phone) use. If you're on a desktop/laptop you just switch to the desktop at boot up and forget about metro.
Actually, I think you are missing the central point. The new UI is not supposed to be "a UI", it is supposed to be "the UI" (with the desktop being the old, legacy, deprecated UI that they had to include because of all the old software that uses it). They hope to get rid of the desktop entirely in some future version, although the odds of it happening before Windows 12 seems pretty slim and it will probably still be lurking around somewhere in Windows 20. As I understand it, they would like to get rid of the Win16/Win32/Win64 GDI type graphics interface entirely - the new UI and its apps don't use it, they use something new instead. As for why the new stuff can't run in a window in the desktop environment, that would be because they decided not to do that.
Also notice how you can not even have it boot directly to the desktop even if that is how you use it. This is a deliberate choice on their part. The Server 2012 version of Windows allows you to boot directly to the desktop (or run with no GUI, for that matter). The regular versions do not. On purpose. (You can get little 3rd party programs that change it over automatically when booting, just like you can get some that add a start bar back in.)
Aside from the UI peculiarities, the underlying OS itself is apparently quite nice. They have evidently managed to make a new version that has less processing overhead than earlier versions when run on the same hardware.