I think certain legalities prohibit free (as in no cash) distros from being what we all want. But for the money, a copy of Linspire is cheap and gets the job done. So does Red Hat and Novell desktops (the money charging ones, that have all of the multimedia support).
Too bad that we all depend on Windows Media and MP3 as much as we do. And that the anuses at the MPAA wont allow a linux binary without pulling teeth. Linspire has a legal one, but Lord knows for how much longer. That deal is set to expire in a year or two.
I think that is a hurdle, but there is definitely no reason why the mass cannot use linux. I think that if there were more desktops at the store that demoed the products better that we would have a quicker adoption.