Metro is terrible as a desktop and tablet UI, but was fantastic for mobile interfaces. Microsoft's mistake was thinking UIs would converge across form factors.
I still believe the overall vision is/was good. They just would have had to make a few more compromises, like some additional optimizations for desktop where users could profit more from mouse and keybaord.
But another factor was that the change was just to deep of a cut, and the world was just not ready for it. People were overwhelmed when they were expected to suddenly use magic corners and gestures instead of clicking a button. If it would have been a slower change, it might have worked out better.
Seems like Microsoft thought if people could stomach the transition from XP to Vista/7, then the transition to 8 would be taken with the same amount of initial resistance followed by acceptance.
There's only so many times you can tell your customers that their hardware isn't up to snuff for your software, whether in terms of computational capability or interaction modality, in the same decade!
But another factor was that the change was just to deep of a cut, and the world was just not ready for it. People were overwhelmed when they were expected to suddenly use magic corners and gestures instead of clicking a button. If it would have been a slower change, it might have worked out better.