I have used many OS in my short life and I feel MacOS X is the biggest difference between PCs and Mac. Security wise, no doubt MacOS X is incomparable. Pretty much everyone I know at work or school has suffered some type of "virus" attack their life time on their PCs - can't say I know or met anyone who had any virus or spyware issues on their Mac.
Speed wise, Windows 7 is just a bloated system. It uses too much resources to do too little - some of the features are even directly copied from Mac. The interface on all three os (XP, Vista and 7) are just simply unintuitive and clunky. You have to click too many places to get simple things done. OS organization is crappy.
Mac OS X on the other hand seems just much more intuitive to me. Very easy to learn. Some PC folks try to hit back with the "you don't have the right click" but that's a lame argument because you can enable the right click on your Mac, if it is not enabled yet.
PCs in my experience tend to crash more, the programs take a long time to start, and there's always the Internet Explorer which is up to new ways to destroy your PC experience. With Mac OS X, none of that. Safari isn't perfect, but it definitely not as hideous and terrible as IE (doesn't matter which version - they all equally suck). Macs do crash, but in my experience not as much as PCs.
From a programming point of view, my Mac was ready to go right when I opened the box. Java was already installed, and I program on vi and before you know, I am flying (or coding). With PC, you have to install few things, change few environment variables, install some IDE before you can write "Hello World". Too much work.
For me the underlying Unix environment is easy to play with than any of the PC's system variables. Installing or uninstalling apps on Mac is also easier, where you just delete a software on Mac to uninstall it. On PCs, for some software, it's just a pain.
Hardware wise, well, Mac laptops or iMacs are just so nicely designed. I can't think of any all-in-one PC system that looks as beautiful, simple and elegant as the iMac. If you have a mac at home, you show it off to your guests, not hide it under your table. Same thing for laptops, they are thin, have powerful batteries, last longer, have nice screens, and are sleek. I am sure you can buy one nice powerful PC, but when you open that new PC from Dell, Toshiba, Lenovo or HP, don't forget to clean the zillions of crap and useless software that comes with that PC which basically cripple your OS experience. That's why many advanced users that I know, like a clean install of Windows 7 or XP on their systems, rather than the crap that comes along with it.
I could go on and on.