The Metro UI is great. The tiles are smart, the apps are beautiful, the solution to multitasking is promising, but this whole new approach cannot coexist with a Windows desktop. I almost fell out of my seat when the demonstrator nonchalantly switched back to the Windows 7-style desktop, complete with start menu, taskbar, and maximized MS Office.
This the old curse of Windows backward-compatibility in a new form – not in system-level cruft this time, but in incompatible UIs from completely different worlds. You can't put a new tablet UI and a traditional desktop UI next to each other on the same device and expect users to use them together.
It's as if Microsoft is attempting its own "Back to the Mac," only they never actually left. Apple got it right, starting fresh with a new form factor and adapting appropriate elements of it to the desktop. Microsoft is also starting fresh with a new form factor, but then trying to shoehorn the whole thing onto the desktop.
Of course, this is just a preview. I'm sure things will change a lot between now and the next preview, let alone the final release. But this isn't starting off on the right foot.