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.
The real problem is not usability (that has largely been addressed), it is software compatibility. Even though software like OpenOffice.org is 99% compatible with Microsoft Office, that last 1% still scares people off.
The thing is, 99% of office workers/non-gamers could switch to Linux without any significant problem. But if they're getting Windows free (pirated), they have little incentive. I am happy to see Microsoft crack down on piracy, as it forces people to accept the true cost of their choices.
I've been using Linux as my sole desktop operating system since 1997 or 1998. I manage to accomplish what I need to accomplish. I had one job where I occasionally needed to run a Windows application (we were using a MS SQL Server db for a Linux-hosted web application). For that, we bought VMWare, and I would just run Windows in there when I needed it.