Need differentiate the points of views:
From the shell-users point of view here is no big differences. OS X is fully certified Unix operating system, so it's command line is fully compliant with Unix standards and POSIX. You have terminal with shell, all shell commands (BSD style), you can install e.g. macports for GNU-versions of commands. e.g. the bundled
sed is without --long argumnets, when you install macports you will get gsed - the GNU version. You have common directory structure as /bin /sbin /usr/bin .etc..
From the X-windows-user point of view - you can use X-windows apps, but it is not the default GUI. OS X has own GUI (called Cocoa). For an good overview you shoudl read this developer doc.
From the point of view system-admin - here is some bigger differences. Many configurations are done with Apple's own tools (mostly GUI) - but all tool has comman line interface too - but is IS a bit different from common UNIXes.
The biggest differences are for developers. OS X using fully object oriented message passing Objective-C. The Mach kernel IS message passing oriented kernel. Developing with xcode (Apple's development tool) is really easy.
I'm after using many different unix flavors over the last 20+ years (starting with BSD 2.9 on PDP11, via BSD 4.2/VAX, XENIX, SCO Unix, Consensys, Linux and FreeBSD) now using OS X nearly ten years. IMHO it is the best UNIX what I used anytime - so therefore the answer is subjective...