Originally Posted by Karelia
Very true. I can't say I'm innocent, either. So let me close with this; why is it important? I assume we can agree that it's miles ahead of both Vista and Tiger in terms of visual appeal. If that's the case, why do the little things matter?
Because the use of 3D space has major, major implications for the UI metaphor, unlike, say, the pinstripes on the menu bar.
The original, "classic" Mac UI was based on some pretty strict rules regarding the behavior and placement of UI elements, collectively know as the "Human Interface Guidelines" (HIG).
By a lot of people's lights, the "friendliness" and "ease of use" of the OS derived from just that-- things looked and acted and interacted in very predictable ways, ways that were, for computers of their day, very "natural" metaphorical extensions of how we interacted with actual items on a desktop.
Certainly I'm not the first person to have misgivings about the seemingly casual relationship to any apparent hard and fast HIG in OS X.
Whether you realize it or not, a carefully considered, consistent HIG that takes into account how human beings actually think, learn and interact with the things around them makes all the difference in the world regarding the usability of a given OS.
That doesn't mean UIs should never change, or that there aren't better ways of organizing the elements of an operating system that take into account new, larger and more numerous file types, or that take advantage of the graphical horsepower of contemporary machines.
But to be truly useful over the long haul, and to contribute to an OS that is both productive and pleasurable to use, such changes must be in the service of a well though out HIG.
Color schemes or surface treatments don't matter much (within reasonable limits) , since they don't effect the all over experience any more than using a desk with a cherry top vs. using a desk with an oak top would (although, as the move to a unified window look shows, too much variation on this count can be a problem in its own right).
But a change from 2D to 3D space, no matter how apparently trivially implemented, is a massive shift in the fundamental paradigm of what the OS "means".
EDIT: If you're interested, there's a sort of semi-famous article
(in mac circles, at least) over at Ars Technica by a guy named John Siracusa that touches on some of these issues. It's primarily about the finder (although "finder" as it is used in the article is a term of broader significance than the app-like thing in OS X), and even if you think he's full of shit it's well worth reading.