Nobody seems to have mentioned the only part of Windows 8 that has truly hindered my productivity.
There are two disparate work areas; the old windows desktop with its task bar and the new "metro UI"... I'd have no real issue with the mix of full screen apps and desktop apps, if they shared a single task area.
I frequently find myself "loosing" a window, for instance by opening Chrome in the fullscreen UI then continuing to work in office in the desktop mode. Naturally alt-tab will allow me to switch back, but I find it very disconnected and arbitrary to not be able use the taskbar to switch between tasks.
Right now the whole OS feels like it's separated into a touchUI and a clickUI and doesn't give me a smooth productive user experience. I'm incredibly disappointed, as I've been really looking forward to Windows 8.