I don’t always agree with Jakob Nielsen (for example, his praise of the “ribbon” UI convention), but I think his criticisms of Windows 8 are spot on:
I do think Metro/Modern has more elegant typography than past UI styles and that the brightly colored tiles feel fresh.
But the new look sacrifices usability on the altar of looking different than traditional GUIs. There’s a reason GUI designers used to make objects look more detailed and actionable than they do in the Metro design.
There’s a reason buttons should look like buttons, and that text areas should look like indented wells. It’s a convention that has been established to let users know what they can and cannot interact with.
Then again, most of those conventions have been established through precedent. Perhaps they are outdated, and there’s simply a shift required. On the other hand, why does the responsibility for adapting to that shift fall on the user?