Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Both Unity and Metro borrow heavily from the mobile world, and for that reason seem likely to appeal to an increasingly mobile-minded world of consumers. As I've said before about Unity, this is a good way to attract mainstream users, particularly when you're trying to help them get used to something new.
What I'm not so sure about, though, is whether these types of interfaces are right for the desktop, and especially for the power users who--“post-PC era” notwithstanding--do still tend to spend most of their time there.
You can, of course, still use the traditional desktop in Windows 8, and you can install something else on Ubuntu. The fact that these new-style interfaces are increasingly becoming default, though, is making me wonder if in all this focus on “post-PC” and multiplatform computing, there may be a growing niche for a less graphically focused alternative for desktop power users.