I was under the impression that the classic Windows UI with start button would be the default, with this more tablet-esque UI as an option? I'm not as informed as some probably are, though, so I may be wrong.
However, as long as this tablet-esque UI is optional, and the default UI is similar to Windows 7, I'll be happy with it. I pre-ordered
Leap Motion, and using that with Metro looks like it will be really, really cool and fun.