OS X's interface is quite well thought-out, and not just because Apple did it.
I actually noticed something quite interesting recently--
My home system has been OSX for about 3 years. I do a lot of work from home, so I use that quite extensively in a "work" manner.
For the last year my primary system at work has been a Linux laptop (Ubuntu, switching between Gnome and KDE).
Prior to that for 3 years my primary work system was Windows XP.
Using all 3 of these OS's extensively, for the same tasks, I have discovered that OSX is actually the least mouse dependent! I was actually quite surprised when I realized that I spend more time on the mouse in Windows than I did on OSX, and found there were a large number of things that were keyboard accessible on OSX:
Application launching (either through spotlight or Quicksilver)
New windows / documents
Closing windows and applications
_task switching_ (emphasized because it is so much easier on OSX)
Searching (spotlight again)
Plus, a lot of the basic functions because the shortcut keys are so standardized.
Windows fails, because for so many of these core functions there is no easy shortcut, sure I can copy and paste, but if I want to launch OpenOffice in OSX its <command><space>open<command><enter> -- to do the same thing in Windows, it would be <windows-key> then a lot of arrows, and maybe some letters to jump to menus. Easier to just go to the mouse.
Linux fails because there is a complete, utter, and total lack of a standard in the shortcuts. Its funny, because you expect Linux to be a keyboard jockey's heaven, but it isn't since you have to memorize so much.
Somewhat related, there is a measurable increase in productivity when I'm on OSX -- my friends notice because they get more daytime email from me (and have taken to guessing when I'm working from home, when I'm working from work, with almost complete accuracy). But at the same time I'm putting in the same amount, to more, work.
It all comes down to the ease of task switching, and nothing handles that like OSX, especially from the keyboard. Exposé, command-tab, and dashboard are an amazing combo.