Normal open market operations swap cash for short-term government bonds, and so affect the short, nominal, safe interest rate. But when the short, nominal safe interest rate is zero, central-bank open market operations are simply swapping one zero-interest safe government asset for another: it is hard to see why they should matter. If the Federal Reserve wants its actions to matter, it needs to swap cash for assets that are not short, nominal, and safe.
Nick Rowe remembers the dictum of Napoleon Bonaparte: "When you set out to take Vienna, then by God, sir!--take Vienna!"
If a central bank is going to buy assets that are not the short, nominal, safe assets it usually holds in its portfolio, it should buy the longest, realest, riskiest things it can find.
"The interesting question, I think, is why the March 2009 announcement was not regarded as credible quantitative easing..."
My answer: because the Fed announced it would buy nominal bonds, rather than some real asset (like the S&P500). And the reason why a real asset would work, and nominal assets can't be relied on to work, is precisely because the signals that people get back from bond prices are so mixed. "Bond prices went up/down. Does that mean that the Fed's policy is working/not working?" Shrugs shoulders, bemused by trying to work out the implications where pushing a policy lever one way causes the lever to pull back the other way. How do you know if the Fed is really pushing or pulling it?