In general, I think Windows 8 is a step in the right direction. It makes sense to combine tablets and notebooks into a single class of computers that run a single operating system.1 Unfortunately, it’s a bit of a stumbling half-step. Much of it is poorly executed; many of the UI mistakes seem unprovoked, and should have been avoidable.
My most-common response to Windows 8, which I unwisely installed on both my Windows PCs, is something along the lines of “oh, dammit.” That’s because Windows 8 has two completely different UI paradigms coexisting. I mostly use the Desktop to get work done just like I have in Windows for years, but it regularly kicks me out into a Metro app that (a) takes forever, and (b) is a completely different user experience. Menus work differently, things slide sideways instead of vertically, fonts are changed, there is no taskbar. So I have to stop and remember how to work things on the new screen, and how to get back to the one I was using before. I think I was right to compare it to Windows Vista, which was an intermediate step to Windows 7, which still rocks. Windows 9 will probably be awesome, but in the meantime, Windows 8 is a mess.
Read “Nielsen on Windows 8” on ignore the code, and “Windows 8 — Disappointing Usability for Both Novice and Power Users” on Jakob Nielsen’s Alertbox.