Excellent New York Times article by Robert B. Reich.

Premise: 37% of all consumer spending is by the wealthiest 5% of Americans, this caused by an increasing wage inequality since 1980. Unless this inequality is reversed (as it was between 1945 and 1980), the country will experience booms and busts due to the inherent volatility of wealth concentration – the rich stop spending, the middle classes have no discretionary income to speak of, the economy stalls.