I disagree with his quote that 80% of people use computers for only web browsing, email,simple office work and to hear music and see videos. I don't know anybody for whom that is true.

80% of people use computers to do those things and one more little thing not covered on that list. And that one more thing is different for all of them. For some it's plotting courses for their next sailing trip, for some it's designing sewing patterns, for others it's some heavy obscure spreadsheet calculations or checking building plans in their GIS software and so forth and so on. Basically a tablet that only let you do web browsing, email,simple office work and hear music and see videos won't replace a computer for anybody.

See also http://www.joelonsoftware.com/articles/fog0000000020.html