Advantages of Mac OS
Mac OS is a two-layered system: the attractive GUI sits atop a Unix core, and Unix is best-known for its security features. It's simply impossible to install a destructive trojan or virus unless the user explicity allows it root access via typing in the admin password. Mac OS's built-in firewall is set up to work unobtrusively out of the box as well as being highly configurable.
Mac OS users should be vigilant about strange files and never allow an application they aren't certain of admin access, but they don't need special anti-virus software.
Because OS X was designed from scratch from the ground up, Mac OS is incredibly stable. Apple controls production from start to finish, so every part of a Mac is designed and tested to work together.
Ease of Use
Apple's known for hiring the best industrial and interface designers around, and it shows in the intuitive nature of the GUI. I don't mean to sound like I'm verging into fangirl territory here; it's just true. Like the OS's reliability, the OS's functionality is designed to just work.
The attractiveness of the interface can't be discounted. Some scoff that looks aren't everything, but when you're spending 8 or more hours a day staring at a screen, it's a relief to spend that time staring at a well-designed screen.
Advanced users have easy command-line access with the Terminal app.
Integration With Apple and Other Products
Mac OS will automatically recognize and work with any other Apple product, such as iPhone, Cinema Display, Airport, and iPad. On top of this, additional drivers are rarely needed for non-Apple products, and I can't think of the last time I plugged a non-Apple device into my Mac that Mac OS didn't recognize.
Mac OS Can Run Windows at the Same Time
Mac OS runs on Intel chips, which means you can run XP or Vista concurrently with Mac OS, with Boot Camp or Parallels software.