Why not Linux?
I have a couple of Red Hat machines here, and like it a lot. But it is not ready for the masses yet, and still needs some geekish playing around to make it work, especially with some hardware, and there are just too many things for "Joe Plug and Play" to learn and adapt to at present.
Ximian is a great email client. Linux represents a lot of work by a lot of people.
Just a little more time.
I am quite familiar with Linux which I use for scientific work. But Linux is not an operating system for the "consumer user" -- a great system though for the enthusiast, or very advanced user, who is willing to put the time in to learn the system. Frankly, if Linux had the aftermarket support that Windows has, or even a reasonable fraction of it, it would be my primary...as opposed to secondary.
I certainly am for the 'open source' initiative, and hope that Linux gains more support with upcoming releases.
I enjoy Linux (Fedora, SuSE and Mandrake primarily), but it's definately not ready for your average home user yet. The system will definately have to be simplified a bit on the UI end before the typical family will be comfortable with it. Gnome and KDE are starting to look real nice now, but the UI's are still geared to hackers (in the traditional sense), geeks, and power-users.
Software and hardware installation processes really need to be unified and cleaned up a bit as well. Having worked in tech support, there are enough people out there that get confused by the InstallShield wizards on Windows that you're just not going to be able to get them recompiling or editing scripts for installations, no matter how powerful the new way is.
If everyone in the world had the hacker ethic, then Linux would be a perfect solution, but most people really don't want to know how everything works, they just want it to be taken care of for them.
Given another couple of years and some real effort at addressing these issues, I'm sure that Linux will be in a more suitable position to compete in the desktop market.