I've said this before, but it bears repeating.
MS was successful selling an OS for traditional PCs using the desktop metaphor. A lot of people liked it. So when it was time to build their mobile OS (WinCE) UI they forced the desktop interface into a four-inch screen, Start button and all. It sucked, and users told them so. Later, they built the Metro UI for phones. A lot of people liked it. Now MS is forcing that tiled UI paradigm onto desktop machines. My prediction: It will suck, and users will let them know by not buying Win 8.
Unfortunately, whenever MS has a hit, they suddenly think that every problem is a nail that they should use that same hammer on.