Besides the obvious stability and "it just works" points;
OSX has a lot of little touches that make an overall great experience. Expose for example is one of the first things I point out to people I convert. When I use my work windows XP machine, out of habit I find myself mousing over to the corner to use expose - obviously this doesn't work. Even Vistas incarnation of the feature isn't quite as useful as you can't see a whole picture of the window.
I also am a bit of a minimalist, so I like how there is one complete program for everything, and its kept simple/uniform. Not to get in a windows vs mac debate, but for sake of contrast, it seems like mac isn't just delivering an operating system, but a whole suite of pretty much every program the <average> person needs - but nothing more. I just prefer the clean and uncluttered look, and it seems like no one else can replicate it. I have searched all over for a photo organizing program thats as simple to use as iPhoto for both my moms computer and my girlfriends parents pc. Other than expensive alternatives, there isn't much. Photo Gallery that comes with Vista is a wreck, and I don't think average users have too much luck with it.
One other big point as said above, hardware design is top notch and usually very high quality. I know die hards say its functionality not looks that matter, but I don't care if you spend $1,000 or $15,000 on a computer, there is no reason all manufacturers cant design a decent looking case. My brand new Dell Latitude Laptop (the work issued pc) looks like it could have been made 10 years ago, less the "guts" of the machine. It may not be the biggest deal, but while they are at it, why cant they streamline it a bit?
There is probably a lot more, but from an average users perspective, those are some of the top likes imo.
- Chris