Try just jamming the mouse into the upper left corner and click there -- without calling up the thumbnails. From any screen in the Metro environment, I believe, that corner will work as a "Hotspot" that goes to the Desktop.

Similarly, in the Desktop environment, the lower left corner goes to the Start Screen -- even if there's no start button there.

All the corners of the screen do various things, depending on whether you left-click, right-click, or just hover -- and depending on what you're in at the time.

In my system (Windows 8.0) all of the following options are available via "Hot Corners":

Start Screen
App Bar
Peek at Desktop
Minimize Windows
Semantic Zoom
Quick Links Menu
Desktop Context Menu