It may be just because I'm tired, but I'm really not sure about how I'd go into specifics.
I won't just parrot the whole "no malware!" bit, because it's usually not an issue on Windows if you're not stupid. It is a little comforting that the chance you'll be hit is lower anyway, though.
And if you ask me, the entire OS is just designed much, much more nicely than Windows is. Apple has spent a lot of time and money researching these things over the years, and while they aren't right about everything, some things just make sense - for example, one static menubar for the whole system vs. the pointless redundancy of attaching one to each application window.
There are also all kinds of little productivity-enhancing bits spread throughout the OS. A good example is the little icon found in the titlebars of windows that contain documents:
Hidden inside that little icon is a lot of functionality. Want to move or copy the file that's currently open, but don't have its folder open at the moment? Just drag the icon like as if it were a regular document. Hold down the alt key to turn that move into a copy. If you're wondering about the document's path, it can also do that. Hold down the Apple key and click to icon to open a menu revealing the document's path:
Of course, clicking any of the directories shown in the list will open them in the file browser.
• The UI is optimized for handling large numbers of windows that are all differently-sized to best accommodate their functions, which contrasts Windows which is designed around all program windows being maximized. It's much more friendly to multitasking.
• Drag and drop is not only widely supported throughout the system, but also in the vast majority of Mac applications.
• Mac applications, rather than sprawling out all over your system like some sort of octopus, are for the most part self-contained and are usually removed by simply deleting them.
• There is no central registry to grow massive and get corrupted; all programs store their preferences in separate files in the users' preferences folder.
• Text appears on the screen exactly how it will appear if printed.
• Nearly every Mac application gets to take advantage of the system-wide grammar and spell checker.
• The system never forces bizarre fake folder hierarchy like Windows does.
• OS X is sold as one package, one edition, one size fits all. Even the 32-bit and 64-bit versions are on the same DVD.
• 32-bit programs and drivers run perfectly on the 64-bit version of the OS.
• OS X requires no serial, activation, or registration and thus won't bitch at you when you swap out system components.
There are a lot of other differences, but I'm tired and thus my brain is dead...
So aside from major design differences, little things like that add up to really make a large difference once you're accustomed to the system.
Another distinct advantage is that you can perform pretty much any generic Unix or Linux command through the OS X terminal. You can also run a very large portion of Linux software in addition to your toolset of native Mac applications.
