I'm going to try to answer this with as few opinions as possible. I'm certain that I'll still come off as opinionated because I do have an opinion, but I'll try to be as factual as possible.
The first and foremost difference isn't noticeable to the average user, except that this difference results in better stability, security, and performance. Mac OS X *is* BSD Unix. They call it Darwin BSD. In fact, if you've ever used Linux, and you're a command line type of guy, then you'll find that the commands on OS X are identical to those on Linux (or FreeBSD or OpenBSD or NetBSD or HP-UX or AIX... you get the idea).
So then all Apple does is take this Darwin BSD and slap their Graphical User Interface (GUI) on top of it. Now, among the differences between Unix and Windows, one of the most important is that Unix has no registry. Again, without trying to give an opinion, the bottom line is that's one reason why Unix far less likely to crash than Windows. One opinion that I will say is that the registry is the single worst mistake that Microsoft made when designing Windows.
Here are some factual problems with the registry:
1) Has a finite size limitation unless you manually change it.
2) Gets fragmented, slowing down your computer.
3) Is cryptic, poorly documented, and difficult to edit.
4) When you uninstall software, it still leaves a bunch of junk in the Registry. This also can happen with those text files in Unix/Mac; however, with lots of text files, extra leftover settings aren't a big deal, but in the Registry--because it's a single database, this causes it to keep growing over time, which can severely affect performance.
5) If a text file with settings gets messed up, corrupted, or deleted, then you've only lost the settings for one application. If your Registry gets messed up, then you may not even be able to start Windows.
Macs don't need to be defragmented. To oversimplify, the reason for that is that they don't let files get fragmented in the first place.
The reason that creating Hackintoshes are difficult is because Apple ONLY makes drivers for Apple computers. If you're trying to make a Hackintosh on a computer with hardware that isn't compatible to Apple's drivers, then you're going to have a bad time. This is a double-edged sword:
The bad news: You pretty much have to use Apple hardware to run Mac OS X, and that hardware is expensive.
The good news: Mac users never ever EVER have to worry about drivers. Never. In fact, if Ubuntu was to control what computers it could be installed on, like Apple does, then it would pretty much be the same thing as a Mac, but with a different GUI. Obviously, you can't run Mac programs on Ubuntu, but my point is that they'd have equal reputations for stability and ease of use.
Now the virus issue. CAN a Mac get a virus? In theory, yes. DO Macs get viruses? No. All the pro-PC guys will tell you that Macs can get viruses. They'll scream about it until they're blue in the face. OK, then where are all the questions on Yahoo Answers from Mac owners who need help dealing with virus infections? Where are they? Hello pro-PC guys... I'm talking to you.
Now all this does come off as somewhat snooty. And, as much as I'd like to get on a soap box and rant about why I favor Macs, let's look at some of the benefits of Windows:
1. Everybody knows how to use Windows, because everybody uses Windows at work.
2. Far more software is available for Windows (disclaimer: unless you want all the latest games, or you have some specialized program, then Mac runs all the software you could need or want).
3. You can install Windows on anything. Every hardware company makes drivers for their products for Windows.
4. Virtually all specialized software is for Windows. For example, I work at a court of law. There is a program that helps judges calculate alimony and child support in divorce cases. No such program exists for Mac.
5. Organizations have better central control mechanisms for Windows. For example, I can go on my server, and set it so that the screensaver for all employees will turn on after 10 minutes or 15 minutes or whatever I choose. I can make it so that everyone in a certain department can't open the control panel. I can make it so that the next time any and all computers restart, Office 2010 automatically gets installed.
I see it like this: A Ford and a BMW will both get you where you're going. Sure, the BMW costs more, but it's nicer, it has cool features like a rear-view camera for backing up, it comes standard with a GPS Navigation system, and it is far less likely to break down. I'll close by saying this: We've all heard stories about people switching from Windows to Mac. I know several dozen such people personally. What I've never heard of is a Mac user switching to Windows.