I switched about five months ago, and the list of things I LOVE about my Mac is huge!
1/ As everyone has noted, the stability and "just works" nature of OS X is fabulous!
2/ No virus', trojans, serious attacks from the dangerous world of the net
3/ Widgets! FABULOUS!
4/ The fact that you can close an application window and still have the application loaded and waiting. Subsequent uses of the application just "snap" into being . I LOVE the responsiveness
5/ The Spotlight integration into the File Save and Save As dialogs. You can save a file anywhere on your system with only a few keystrokes instead of all that tedious navigation through the file system that Windows would impose on you.
6/ Open Source and Unix! Mac OS X is built on a FreeBSD base, and thus supports a huge array of Open Source Software, via the DarwinPorts or Fink bases. I switched from a combination of Windows and Linux to Mac OS X. I now have the best of all worlds, having installed all my old Linux favorites via DarwinPorts, plus all the Mac OS X stuff.
7/ Encrypted virtual volumes - it is DEAD EASY to set up an encrypted virtual volume (a .dmg file) in which to keep your sensitive records - financials, wills, etc. It costs big money to get a program to allow you to do this in the Windows world.
8/ Included Software - iLife is wonderful, even the basic image viewer, Preview, is rich and feature packed. No such stuff on Windows!
9/ Spring loaded folders - what a GREAT idea and so easy to work with. I have to work with Windows at work and I really miss these there.
10/ The little dot that shows up in the red close button on each window, letting you know that there are unsaved changes. Its a little thing, but really, really nice.
11/ Installing apps. Apps are basically self contained packages on Mac OS X. Installing is dirt easy - just drag to the Applications folder, and applications don't strew hundreds of little DLL and other support files all over your disk. Much, much more sensible!
12/ The integration between Safari and Mail. I LOVE the ability to email and entire web page (not a link, but a copy of the full content) with a single command. Did you know about this - go to a page you like, and type CMD-I. Boom! Mail opens up and presents you with a message ready to be mailed, whose content is the page you were just looking at. This is GREAT for mailing travel itineraries, or even just a story you saw on a web page that you want to share.
The list goes on and on, but I will stop here. Apple and Mac OS X are fabulous. Windows... well, Vista better be really good if it wants to keep up, and I doubt it will; Leopard will up the ante so much that the team in Redmond will be almost instantly behind the eight ball again.
Rock on Apple!