Keynesian (and some Monetarist) versus Austrian views on the business cycle revolve around two basic empirical issues: the nature of capital and production and the nature of the national budget constraint.
Concerning capital: Ironically, Keynesians appear to assume a kind of “equilibrium always” story in which productive (human and physical) resources can, at low cost, be simply shifted to and plugged in to the production of whatever product or service happens to be the object of demand stimulus; and, further, that if enough of this is done, income, employment and tax-payments will rise precipitously. In this way economic-policy can produce economic growth that will expand the budget constraint sufficiently to be able to pay off the large past deficits (the accumulated national debt).
Austrians maintain that both of these (connected) assertions are false. Productive resources are not fungible. They cannot be easily moved and reassembled to produce whatever has been arbitrarily stimulated. The formation of productive resource combinations is the spontaneous result of idiosyncratic entrepreneurial judgments about future demand and revenue. There is nothing mechanical about it. Amorphous stimulus spending is not likely to produce an environment which entrepreneurs are willing to bet on. Rather, the result will be a malaise of uncertainty and reluctance to borrow and lend. The intertemporal budget constraint is real, not only for private citizens, but for the nation as a whole.
We cannot spend our way out of debt by hoping that the spending will lead to growth, especially when those doing the spending have no idea how to plant the seeds.