My problem with these "just work" descriptions is that people have very different needs.
I use both Ubuntu and Mac OS regularly. The things that I need to "just work" are a lot of programming tools (gcc, python), databases, and servery stuff like databases, web servers, etc. Getting those to work on Mac OS is unpleasant. MacPorts and Homebrew are both terrible in comparison to the APT world. "apt-get install apache2" is very much "just works" in my book. On the Mac, I'm fine as long as I use Xcode and other Apple-specific tools, but anything else ends up being frustrating.
People complain a lot about desktop choices for Linux, but I never found any of them any worse than Mac, and some are better tailored for certain workstyles than others. All the major ones (GNOME with Shell or Unity, KDE, XFCE) are mature enough now for everyday work, even if they weren't so a few years ago. I really find all of them easier to work with than Mac's desktop. I don't like Mac's bubblegum dock, and I find the Finder to be perhaps the worst file manager ever made.
Another aspect of "just work" is installation. Installing a free OS can be painful on some hardware (and trivial on others). Since you can't (easily) install Mac OS on non-Apple hardware, this problem doesn't exist there, so it indeed "just works" in this respect. If you want a "just works" experience with a free OS, just buy a machine from System 76, a truly wonderful company that has yet to disappoint me. Comes with Ubuntu and everything working, great hardware and great support. And for me, all the things that I want to "just work" indeed do.
I use Windows 7, too, and it's fine, but I really need the Unix stuff to do my work.
Can we retire the "just works" phrase, or at least find better ways to qualify it?