OS X has been a really innovative force in personal computing. It's highly usable, lots of different kinds of users are able to work with it. It's compatible with lots of different standards, and it provides a lot of tools to developers that make even the sucky third party software pretty nice. I think if you look at Windows Vista, and the latest versions of KDE and GNOME and some of the other open source user interfaces, it's pretty easy to see some resonances of OS X.
More importantly, probably, it proved that Unix and Unix-like operating systems were viable and usable for desktop use cases. While we've been able to run BSD and Linux on home computers for years, I don't think we've thought of Linux as being something that anyone could run without needing a lot of technical background.
Ubuntu Linux followed this trend, pretty persuasively. Ubuntu makes desktop unix-like experience possible. Which is a really big thing. Chris and I are both using Ubuntu these days for our primary desktop computers, and it's been really interesting to compare notes. One thing that we keep coming back to, is that despite the fact that the core of the OS is great, the user interface (UI) is tragic. OS X proves that it's not only theoretically possible to have a nice UI, but it's possible to do that on a unix-like system.
As an aside, I'd bet good money that Apple has an in house version of Aqua/Cocoa/Carbon/CoreServices (all the UI and application frameworks that make OS X, OS X) running on the Linux Kernel. Betcha.
And by tragic, I don't mean that GNOME and KDE are unusable, but they're flawed. GNOME doesn't use space efficiently, it's applications are functional but not exceptional (and because of the way the GNOME project is there aren't many 'third party' alternatives), and it feels sort of behind the curve. It works, and it does everything that you might want in a graphical user interface (GUI) but it's not exceptional.
Thankfully KDE fails for completely different reasons. It's attractive and usable where GNOME isn't, and the interface is unique and exceptional where GNOME feels stale and aged. But the applications aren't nearly as compelling, and it suffers from having an interface/look that's too flexible, such that it's pretty easy to get a setup that looks like crap. Not to mention the fact that the kind of rich GUIs that KDE emblimizes don't mesh particularly well with the mostly hacker audience that Linux (and it) attracts. But that's a larger critique of the GUI paradigm, which isn't quite on topic.
So where does this leave us?
I'd say the biggest shortcoming of linux systems is the window management options. I like Awesome and I think there are a bunch of people who might really like it--but it's not for everyone, and I'm admittedly not up to date with enough of the other options to provide a really clear analysis, but I know that this is the the next big issue for open source operating systems.
I'm not sure that I know enough