Chris Pirillo’s Father Tries Windows 8
[I]t’s an eye-opener regarding the risk Microsoft is taking by making essential UI navigation elements hidden until you hover the mouse in the right spots. People navigate with their eyes, not by scrubbing the screen with the mouse.
From the video, it seems like Pirillo's dad couldn't figure out how to go back to the Windows start screen. As far as I know, you do that by hitting the Windows key. So it's a bit like people using an iPad for the first time, trying to figure out how to get back out of an app to the home screen, and nobody telling them that they simply have to hit the hardware home button.
Yeah, you have to be told that this is how it works, but once you know, it's hardly a major usability issue.