OS X works only on a limited set of hardware. I can't install it on any netbook. I can't even legally run it on any new laptop under $1000. The vast majority of people who install linux have 0 problems on thousands of different kinds of computers from a decade old to new netbooks to supercomputers.
Moreover, OS X also has numerous problems with peripherals. I bought a camera, standard usb camera sold by the millions, and there are no OS X drivers for it. I spent hours scouring the web and installing various programs into this location and that location. Supposedly those drivers support some cameras, maybe even mine. But no, to this day I cannot use skype video on my Mac Mini. On my Ubuntu partition, it works with 0 configuration. Plug it in and I'm done.
Another example: a shared printer. In Ubuntu jaunty, there's a brilliant dialog that automatically finds and sets up a shared printer with a few clicks. My friend Jeremy tried to set up the same printer with his Macbook Pro, and he asked me, we poked around, couldn't find it. We got a cryptic error message that said something about 'could not find bla...'. We tried for a few hours searching online to no avail. Today, he goes upstairs to physically plug in the printer directly every time he wants to print.
He tells me that for most Mac problems you have to spend a few hours searching online and you might find some blog post where someone mentions a trick. No, OS X is where Linux was 10 years ago when I was using Debian. Worse, because Debian always had apt-get, so I had lots of good free software at my fingertips. Ubuntu is far ahead in terms of being painless, and it improves every 6 months...