Wealth, and the basics.
Until every citizen has preventative medical care, an opportunity to acquire as much of an education as they would like, and an opportunity for decent housing, no person in the United States should have a net worth of more than, say, $10 million.
Take care of the basics for all citizens and then let them acquire more wealth. But people only need so much.
And as for how much they need, $10,000,000 is plenty. It's obscene for the US to have the child mortality rate it does while people are wealthy beyond what they could ever spend.
Arf