How Linux Could Become the Next Killer Desktop: "Unfortunately, you can only run OS X on proprietary and relatively expensive hardware.
Instead of emulating Windows, Linux desktops should become compatible with OS X, thus creating a great desktop, fueling the growth of both OS X and Linux, and bringing in more developers to both platforms." [osOpinion]

Of course, a lot of the Mac OS X usability stems from the tight integration of software and hardware. Also, may of the Linux application developers seem to target Windows users, making a lot of the same usability mistakes which Microsoft did.