Read Brad’s whole post.  My broadly similar take sees the post Keynesians as having promoted several important ideas:

1. Price and/or wage stability can be destabilizing; Keynes (sometimes) presented stickiness as a policy recommendation rather than a necessary fact.

2. Financial fragility, a’la Hyman Minsky.

3. Noise traders matter for macroeconomics.

4. Moving Keynesian economics away from the consumption function and IS-LM.

5. Behavioral and psychological imperfections are relevant for macroeconomics.

6. They resurrected interest in the idea of an endogenous money supply.

I’m not entirely on board but overall that is a strong list.  These ideas have now been incorporated into the mainstream, but only recently, typically in the 1990s and sometimes by Brad himself. 

The post Keynesians went wrong, however, in several ways:

1. Too many of them obsessed over incomes policies.  On "public choice" more generally they were naive.

2. Too many viewed Thatcherism, the war against inflation, and other developments as instruments of class warfare, designed to redistribute income from poor to rich.

3. They held too many debates over "what Keynes really meant" and cared too much about distinguishing themselves from mainstream Keynesians, whom they often treated as their worst enemy.

4. They ascribed too much power to the liquidity premium on money, and thought it was easy for an economy to end up in a corner where no one wants to produce new investment assets.

5. They failed to model the complexities of system-wide wage and price stickiness, and the differences between real and nominal stickiness, a’la Romer (ReStud 1990) or Caplin and Spulber.  For this reason at some point they stopped coming up with insights on stickiness and fell far behind the rest of the profession.

6. They failed to develop an empirically progressive research program that kept pace with mainstream macro.

7. They failed to realize that the future of Keynesian economics lies in the elements of Milton Friedman — nominal stickiness is real and the action is with monetary policy — that were most Keynesian.

Overall post Keynesianism has been a far more useful heterodox thread than Sraffa and neo-Ricardianism.

1. Sraffa (and more here and here) wrote the seminal paper on the size of the firm under perfect competition.  If contracts are perfect, what does a firm mean anyway?  He doesn’t quite get to that point, but he did understand that if all the cost curves and demand curves are horizontal, we cannot say how small or how large an individual firm will be.  For 1926 this article was amazing.

2. He wrote an excellent 1932 review of Hayek’s business cycle theory.  First he questioned the idea of a "natural rate of interest" as a base from which the market rate might deviate.  He pointed out that there is an entire schedule of natural rates.  His deeper point was that monetary policy-induced resource shifts were not necessarily unstable and due for costly reversal.  The newly-favored sectors might simply keep the resources they had captured through seigniorage, "forced savings," and other non-neutral monetary effects.  Oddly the economic pessimists, usually on the left, have forgotten these points in recent times.

3. He wrote a beautiful introduction to the collected works of David Ricardo.

4. In 1960 he published the neo-Ricardian cult tract The Production of Commodities by Means of Commodities.  Sadly this book is almost entirely misleading.  It tries to resurrect the Ricardian idea that costs alone determine price (or perhaps I should say it cryptically hints at that idea without actually defending any real propositions at all).

At first glance cost-based theories seem plausible.  Draw a horizontal cost curve, as would arise from constant returns to scale, and demand will not matter for price.  Are constant cost such an unreasonable assumption?  Can we not in principle multiply every economic input and receive proportionate outputs?  Not really.  There are almost always fixed factors and the "pure multiplication" thought experiment is not very relevant for price in the real world.  Both Marshallian scissors matter.

Now this was not Sraffa’s argument, but Sraffa’s argument was far more complicated and also worse.  It relied on aggregation theorems which simply didn’t stick, and Mark Blaug tore it apart.

Sraffa also had important relationships with Gramsci and Wittgenstein, but those are topics for other posts, not necessarily by me.

Overall the guy’s output was quite sparse.  He was brilliant before the War but his major book was headed in the wrong direction altogether and it should be taught as a mistake, not an alternative.

Here is the scoop.  John Tierney, citing Ron Bailey, disagrees; I score this one for Tierney if only on public choice grounds.  Key argument:

The prospect of a cap-and-trade system in America has already set off a lobbying frenzy in Washington by industries hoping to write the rules to their advantage.  Given legislators’ eagerness to please their hometown industries, it’s easy to imagine them being just as generous as European politicians.  By contrast, a carbon tax would be more straightforward – simpler to establish and enforce…

Read Greg Mankiw as well.