There's the 47% of people (more or less accurate) who consistently support President Obama. Obviously, that number includes a lot of effete elitists who pay income taxes. Even capital gains taxes for the high rollers.
Then there's the 47% who currently don't pay net income taxes, in part because of the effects of the Great Recession. The number was about 40% pre-crash.
Then there's the more general and longstanding argument about the hoard of moochers who want you to pay for their baseball tickets and the tab at Bennigan's and maybe even some crack with your tax dollars. Most Republicans leave the number of these people intentionally vague, so as to expand and contract based on an individual's sense of social resentment and rage.
But somehow Romney took these each together and rolled into a "half the country is made up of mooching losers who won't take responsibility for themselves and they want to elect Obama so he can take your stuff" joint.
I had a number of emailers last night who wrote in and said some version of, "Hey, you must not get out much. Republicans say this 47% thing all the time." But again, that totally misses the point. If Romney had simply said, 47% of Americans pay no income taxes (which I think he has actually) and that's wrong, I don't think he would have had that much problem. Expressing such unvarnished contempt for these people as victim-feeling losers who can't get their act together and equating them one for one with all Obama supporters put him into altogether different territory.