just a thought. I'm not knocking Linux at all, but I think something that would really help it on the desktop is if someone were to develop a user friendly UI layer the way Apple did over Darwin.
Instead of using X11, use a built-from-scratch, modern graphics framework (display Ghostscript? an open source Quartz?), develop a Linux-based OS that's as full-featured as Mac OS X, using KDE or Gnome as the UI foundations.
For it to be accepted on the desktop, much of what makes it UNIX will have to be hidden to the user. They don't know, don't want to know, and just don't care about anything in /etc, or /dev. they just want things to work. The concept of "mounting" drives will have to be an automatic process. You put the disk in, and it's mounted for you.
They're making huge strides in usability, but unfortunately, not enough. Remember, that for it to make it on the desktop, it can't be "good enough" or "as good"... it has to be BETTER than Windows. Better in every respect, not just licensing and stability. Usability has to be ahead. The Linux world will have to break down and accept processor-wasting eye-candy, because users are accustomed to it now. It's what they want!
Make a GNU implementation of the Apple/NeXT packages so that a program and all its support files are contained in their own tree, and represented by one icon. Users can't be bothered with worrying about library dependencies and what folder a file went to. It should all be contained in one icon. This is easier than Windows. Drag one icon to install the thing. No extra files, no registry, no endless installers. Just boom!
I have no doubts Linux will kick MS's ass on the desktop... but they've gotta do more than just make it look like Windows!