Ray Dalio's Bridgewater On The Fed's Dilemma: "We're Worried That There's No Gas Left In The QE Tank"

...The basic issue is that quantitative easing is a much less effective tool when asset prices are high and thus have low expected returns than it is for managing financial crises.  That's because QE stimulates the economy by (1) offsetting a panic by providing cash to the financial system when there's a need for cash, and (2) by raising asset prices, and driving money from the assets they buy into demand and investment, creating a higher level of future economic activity.  So, the policy was particularly wise and most effective (in the sense of impact per dollar) at the height of the financial crisis when there was both a desperate need for cash and when extremely depressed asset prices were heavily weighing on demand and investment. 

Now, there is a flood of liquidity and asset prices are high relative to underlying fundamentals.  So the impact of additional asset price increases on demand is much less (as high asset prices and low future returns make assets more interchangeable with cash).