As I've experimented with my new iBook, I've come to the conclusion that Apple's design philosophy stems heavily from an emphasis on simplicity. A good example is the safe sleep feature found on the newer PowerBooks. Instead of offering separate hibernate and sleep modes like Windows and Linux do, Apple offers one mode which transforms into the other mode as necessary. Essentially, I don't want my computer to "sleep" or to "hibernate" I want it to stay where I was until I come back for it. If that's 5 minutes or 5 days, the computer should still do what I want. I shouldn't have to decide in advance how long I'll be gone.
Another example of the simplicity is the single button mouse. I disagree with this simplication since I efficiently use all three mouse buttons on my Linux machines (when I can't do it from the keyboard that is). Nevertheless, it seems that Apple determined that it was possible to get by with only one mouse button so they did (at least, until recently).
The layout of application screens is a third demonstration of simplicity in OS X. This article about KDE learning from OS X makes some interesting suggestions, particularly, points two and three about simple toolbars. There is no need for a program to display 30 icons for features that one rarely uses (talk about iconic sprawl). True, one can customize the toolbars in most operating systems, but one shouldn't have to reclaim real estate from the toolbar monster.
Apple reminds me of our new house shortly after we moved in. The previous owners had filled the house with all manner of clutter. When they moved, we removed all the clutter they left behind. We put infrequently-used items in closets. Anything we use less than monthly, we moved to the storage shed. Anything we use less than yearly, we donated or threw away. When the previous owners came by to visit, they commented on how much we had done with the place. Of course, we hadn't changed anything but throw out the superfluous junk. Apple seems to have applied the same principle to software design.
All this reminds me of the design philosophy behind 37 Signals: "Our products do less than the competition — intentionally." Or perhaps the philosophy of Unix utilities: "do few things and do them well." Overall, it's been a reminder to me to balance simplicity and functionality in my software design. At some level, all good solutions are simple and naturally apply themselves beyond the original problem domain.