Low Keynesianism: C+I+G+NX=Y; aggregate demand; fallacy of composition and paradox of thrift--attempts to increase aggregate saving have no effect on saving but reduce total demand. Task of government to adjust G to offset fluctuations in I and maintain full employment.
Middle Keynesianism: The interest rate i is supposed to equilibrate two markets: on the one hand, it balances supply and demand in the market for liquid balances; on the other hand, it (with expected inflation subtracted from it) feeds the real economy the market's estimate of the rate of time preference, and so governs the level of investment spending. the interest rate i that balances supply and demand in the market for liquid balances would be the interest rate that balances aggregate supply and aggregate demand in the market for goods and services in general only by the purest chance. To respond to this market failure--to make it so that conditions in the money market are such that the same value of the interest rate equilibrates supply and demand in both markets--we have the Federal Reserve, a powerful organ of collective planning and self government presided over by the World's Most Powerful Objectivist Philosopher.
High Keynesianism: Weirdness about how attempts to pierce the veil of time and ignorance involve not risk but Knightian uncertainty, and about how the theory of aggregate demand needs to be connected with Sraffian insights into the pricing of commodities produced by means of commodities. I have some High Keynesian leanings myself (the first branch, not the second branch), although I prefer to express them in the Shleifer-Vishny "limits to arbitrage" and "inefficient markets" language, rather than in the Davidsonian language that I have a hard time understanding.
Brad DeLong