All right, I will bite because I just switched.
From linux to mac actually.
1. Availability to commercial software. Office and the games available are the two biggees for me and yes there are more software available for XP but more is not necessarily better. Want proof? Check out Office for the Mac the way the damn suite should have been made to begin with. Sure, there are more games for windows but there are a lot more games for the macintosh than linux that is for sure. Its nice waiting on Civ IV right now.
2. The passion. Listen windows always seemed like an OS made by people who did not really care. mediocre. Linux you could see the love but thousands of developers all around the world on thousands of projects all being put together by hundreds of different distros and it all seems like passion disjointed. On Mac OS X its put together well and you see the passion.
3. Quality of Design. Yes, there have been build/manufacturing issues but the quality of design is outstanding from the front row remote to the backlit keys back to the aluminum case and the magsafe connectors. Quality.
4. Built on unix. No matter how many cygwin unix/win tools you get its not the same. Sure, the command line is a bit different than linux but that is the way it is for any *Nix each one is a bit different. rsync backups are just nice and perl scripts work right and its nice. Commercial desktop OS on top of a real unix.
5. iLife man it just beats the ever living crap out of anything I have seen for linux or Windows. I mean its just incredible and when they say it works just like iTunes they mean it. Easy to pick up.
6. MS Office works better on a mac. I thought that was just zealot talk from the mac faithful but its true things are put together with more care and if they put this much care into all their products then people would respect the company as a whole a lot more I think.
7. It just works means something in the Mac world. Hooking up to wireless? Drop dead easy. Hooking up to my samba server downstairs running Ubuntu? Connecting to my printer shared out on another machine? Easy. No fussing around.
8. Integration between the OS and the machine. The same company that made the OS made the machine so the OS does not feel like some tacked on extra with bits that kind of work or only work through a tool that feels nothing like the rest of your tools on the OS. Or in linux where your laptop multimedia buttons don't work at all.
9. Quality of the OSS apps available. Sure, not everthing on linux is available cocoa style or X11 style on the mac. Sure, most of the best cocoa style OSS apps are at least somewhat based on linux derived code but
10. The customer support. Listen I did have a build-quality issue with my first macbook pro. I got it declared DOA and returned. The support folks were great it arrived under the expected time and in general the experience was annoying but the service was great.