Well, Linux itself has to be very tightly controlled. Nothing makes it into the kernel without a proper project leader (who is Linus Torvalds I believe) - it's used on too many high-priority systems to be a security risk, and if we wanted to, we could cut bits out of the kernel and compile it ourselves if we don't like the look of a certain piece of code. How can we trust large multi-national conglomerates to not leave vulnerabilities in systems? Microsoft have a deal with the British government, something that lets investigators bypass security measures employed by the operating system to give the feds a good look inside all your personal files with a USB stick. Granted, if the Police want to see your hard disk contents, there'd better be a good reason for it; but say your computer had been used remotely for naughty purposes without you knowing. There'd be all the proof in the world that it was you, and no evidence to say it wasn't someone else operating under your name. What's to say that Apple aren't going to do something like this in the future without being able to see exactly how the operating system works by viewing the code?
10% market share is pretty outstanding, it has to be said. But will we all be saying the same great things about Apple when it reaches 60%? One of the reasons that Mac OS sells so well is because it's secure and only has a small market share. When it gains a hell of a lot more popularity, we'll be seeing all sorts of horrible code being written for the Mac, taking away one of its selling points. Then again, high market share is a good thing too. What I was getting at about the "generic x86 PC" was the fact that the hardware is just a stock PC motherboard in an Apple case. You can buy a single board fanless PC for about $90USD. Bring your own keyboard, mouse and monitor... and you have yourself a potentially very cheap Mac Mini. It all depends on whether or not Apple wants to have a successful OS or not. Give it time, Apple will eventually release OS X for normal Intel motherboards, and they'll gain MUCH more market share, as customers would only need to install it on the PC they just bought, Whendoes-Tax free. Sure, they'd lose out on the £899 hardware sale, but if they sell OS X for £99, they would have many more users buying an awful lot more of Apple software for use on their non-Apple systems. They'd make a fortune by selling OS X for use on all generic x86 PC's, not just their own.
Of course, I still think that Mac OS is one of the most elegant and simple operating systems I've ever used, but I do have my doubts...