Are children, the unemployed (hard to tax no income), the elderly and are getting social security benefits (which don't get federal income taxes - this actually accounts for roughly half of the 47%), and - as mentioned - the genuinely poor who get more back in rebates than they paid in.

Note that most of these people DO still pay taxes - payroll taxes (SS tax and medicare/medicaid), state income taxes, property taxes, and sales taxes, to name a few.

So to address the original point - the reason that this is such an "outrage" is that income tax is the only genuinely progressive tax that exists in our country - every other tax falls more heavily (in terms of a percent of their income) on the poor and middle class than it does on the upper class. So yes, 47% of the population don't pay federal income tax, but this isn't necessarily a bad thing, and in fact could be said to be by design. <insert quote from The Wealth of Nations here>

They still pay a larger percent of their income in taxes than I do, and a hell of a lot more than many of the ones who are complaining about this fact.