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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.
In this screen shot, taken from an Amazon demo at PDC 2003, Jim Allchin showed off a rather flashy Amazon browsing experience, which utilized the preview pane and one of many 3D views. (To note, this demo was meant to also show off some of the underlying services that were being baked into Longhorn.) The theme you see here is Slate, which was rather hideous compared to the beautiful Aero theme showcased earlier. It was merely temporary.