How are you defining "mainstream"? Because in the context of the Linux Desktop, my experience is that people always define it to mean "someone who is exactly uninterested or stupid enough to win me this argument". Resist the urge to target some nebulous "mainstream" user because you'll just end up with another internet kiosk.
What should be being targeted are the kinds of people who use desktop computers for work and hobbies and actually like computers. Some will claim that that's what Linux Desktop does, but the evidence says otherwise. If you aren't a webdev or OSS zealot, chances are you aren't using Linux Desktop. Personal computing is a hobby I grew up with since the 80s and I don't use the Linux Desktop. There are people who've been in personal computing since before that who are outspoken critics of the Linux Desktop.