While Mac OS X has a UNIX layer under the hood, the
GUI is where it
is less than compatible. While X-Windows is available, and so can
run UNIX GUI applications, most Mac users don't even have it
installed. Most Mac applications are written for the native Mac OS X
GUI.
While most UNIXes have X, X isn't necessary for UNIX. I have (and I
know most people on the list probably also have) several Unices where X
isn't included, one UNIX where X is optional (standard display is
NeWS), and two Unices with something completely different.
I will confess to happily running Linux and OS X, and Linux makes a
darn good can opener for assorted disks &cet. The number of different
UI toolkits (and desktop environment support libraries/etc) that you
wind up installing is somewhat outrageous, especially when you consider
that many of them seem to be pretty much copying the UI of Windows.