My pet theory about this is that 10.0's look was as much as about setting a clear distance between itself and the old Classic as it was about real innovation. (I'm talking only of the UI look).

When Aqua was presented people loved it people hated it but nobody could ignore it. The first time you installed Mac OS X made you feel like you were in a whole new world.

Of course it wasn't that practical and funnily most of it has reverted back... before they pinstripes got faded, then we got the metallic grey, and now just a nice subtle grey.

All that is left from those old days are the traffic light buttons, the gem-blue scroll bars, and the standard button look.