Mac OS X is highly configurable if a person takes the time to learn the commands. But most people don't want to spend time configuring their computers. They just want their computers to work without a lot of fuss. And I would say that the Mac does that better than any system I have ever used.
B.S.
There are many, many things that you can configure in, for instance, KDE, that you cannot configure in Mac OS X.
For instance, window focus policy.
People want their computer to “just work,” until it doesn’t work like they want it to work.
The UI has quirks that are not necessarily the best solution, like leaving a program running after closing all of its windows.
Meanwhile, Apple is changing fundamental behavior in Lion, like how scrolling works.