Aside from the Mac/Powerbooks holding the title as most aesthetically pleasing laptop on the planet, I like the ease and elegance of OSX.
If I had to choose a favorite feature, it would be how completely uncluttered the OS is with third party software. In my view, the best feature is the ease in which the OS can be reloaded to the user's specifications--very simple and intuitive to configure, partition, and customize.
Even with Macs, I always like to reformat, and load the OS on a fresh install with a partitioned HD to my specifications.
Here, OSX is 100 times more efficient (truly, everything "just" works) than XP/Vista!