Well it's true that it's Unix, but as you probably would see from following Unix over the last 10 or so year, various flavors of Unix (Linux, FreeBSD, etc.) have graphical user interfaces. For instance, I'm now posting from a Linux machine using the Gnome environment, posting from the Firefox browser. I also run Windows XP in a little Window through a tool called VMWare, but that's another story.
So no, the average OSX user rarely would open a command shell and "muck around," but a Windows XP user would rarely do the same. Only "advanced" users do this sort of thing.
That being said, OSX has one of the most professionally-delivered and beautiful interfaces I've seen in the scores of OSes I've seen over the years. It is not only simple to use, but it is powerful. The graphical enhancements and effects are quite amazing. It's hard to not be impressed by fully scalable PDF icons and terrific font smoothing.
As for your being sure that Apple isn't going to let third-party programs do X, Y, and Z, I'm happy to report that you are wrong there. We programmers can do anything we like to the OSX interface. A good buddy of mine has replaced the Dock with a folder-based application launcher, for instance. There's nothing really stopping you, from a system architecture standpoint, of rearranging or customizing just about anything on OSX, trust me.
It's ironic that you think Windows XP is more customizable than OSX. In OSX, you can customize anything except for the window manager. In Windows, that's the only thing you can customize!
I could go on for days about my first-hand experience programming on, supporting, and playing with Mac OSX, but I don't really see the point in trying to explain a richly complex OS. It's better that you just try it out some time.