Justin Fox has an interesting commentary or analysis at Time.com:
At its core, supply-side economics is the economics that reigned before John Maynard Keynes came along. You could also call it traditional economics, neoclassical economics, or mainstream economics. It assumes that people respond rationally to economic incentives, and unfettered markets arrive at something close to optimal results. Saving, in this worldview, is a good thing–because savings are always put to use in productive investments that make the economy grow.