Longhorn will bring 3D to the desktop. This means that windows will be hierarchical 3D objects that can be, for example, swivelled aside to make room on the desktop without hiding the window's content. Window contents are 3D so buttons and controls are swivelled when the container window is swivelled.
I am not yet convinced that Longhorn's 3D window manager has much non-geeky values. In the end, we are still simulating a deskbound world and therefore the GUI is constrained and contorted by the desktop metaphor. Because our world cannot be translated into a language that consists purely of desktop lingos like documents, folders, pens, and magnifying glass, we invented windows.
While I could go on and on about the limitations of the window metaphor, I am running short on time so I'll just get to my point, which is:
Longhorn desktop itself should swivel down to reveal a 3D world within which we can work 'directly' with persistent objects instead of limiting ourselves to windows.
Instead of squeezing real world size objects into spacially meaningless hierarchy of folders, lets place them in rooms, buildings, and places to which we can navigate in.
As an engineer, I realize fully that this sort of change is equivalent to a mindless auto executive telling the engineers to lower the dashboard by 5 inches. But this subtle change will finally let computer users leverage their real world knowledge and experiences when working with computers.