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The advent of Windows Flip was a Longhorn-based implementation of Alt+Tab. Not only were you shown thumbnails while you tabbed through your selections, but all the selections would show in larger windows in the background, thus allowing you to see even more clearly what you were selecting through.
Windows Flip was a 2D visual effect, but with the most demanding version of the Aero theme enabled, one could enjoy Windows Flip 3D: a feature that still exists in Windows to this day. Instead of Alt+Tab, pressing the Win key + Tab makes all running tasks fold back into 3D stacks you can scroll through. It's primarily a flashy effect, but when it was first mocked-up, it was a nice change of pace that fit well with the Longhorn vision.
For years prior to Longhorn, Microsoft had messed with the idea of 3D space on the desktop. In the small desktop screen shot in the lower right-hand corner of the image above, you can see what was perhaps the most drastic conceptualization of a 3D Longhorn desktop. In it, the wallpaper and sidebar fold back out of view, much like a door opening away from you, to reveal a sort of 3D space that housed a few ideas, including revealing another desktop or simply showing more information on the current desktop.