With the original iMac, Apple realized that computers could be subject to fashion, not just utilitarian beige boxes. The thing to realize is that the user interface is no different. It’s also a thing of fashion.
User interface cycles between Lickable and Elegant:
Lickable: colorful, 3D, stripey, curvy (like the original iMac).
Elegant: subdued colors and grays, gradients and solid grays instead of stripes, more use of black, flat instead of 3D, more straight lines, fewer curves.
Lickable is obviously the original Aqua interface. Elegant is NeXT. At the moment, OS X is in between the two. I predict that we’ll be firmly in NeXT-land when Leopard is released.
If you want to get a feel for Leopard, install WindowMaker.
(I’m not saying it will be a clone of WindowMaker, just that you can get some of the feel by checking out WindowMaker.)