The difference is that Windows 8 is an upgrade that will replace older versions entirely. Apple's users who didn't like iOS could stick with Mac OS X, but getting back to old-style Windows from Metro looks tricky (if it can be fully achieved at all). The Classic to OS X transition is a much closer match, except that Apple didn't really attempt to change the way you interacted with the computer at that time (it was/is still basically a WIMP interface).
That's not to say that Microsoft will fail, or that Metro will be worse than the Windows 7 GUI. It's certainly going to be more interesting than the average OS launch, but my gut feeling is that the reaction to this will match the reaction to the Office ribbon.
That's not to say that Microsoft will fail, or that Metro will be worse than the Windows 7 GUI. It's certainly going to be more interesting than the average OS launch, but my gut feeling is that the reaction to this will match the reaction to the Office ribbon.