Guardian writer Charles Arthur wrote up a list of the features he loves most in Mac OS X, and pointed out that very few of them are the kind of thing that Steve Jobs shows off in his keynote speeches.
Among his favorite features are simple things like system stability, searching PDFs in Preview, Applescript, parental controls, and Bluetooth synchronization.
I find a lot I agree with in this list. Like Charles, I prefer Camino over Safari; like him, I have little use for Dashboard (I still think a browser is just as good, and more convenient, than most widgets I might want to use), although I do make occasional use of Expose, Spotlight and Automator.
When I stop to think about it, I use OS X because, in my view, it offers the simplest way of doing general-purpose computing. I like keeping things simple, and my experiences with other operating systems have taught me that generally speaking, they don’t manage to make my life in front of a keyboard as simple as OS X does.