Couple more thoughts
1/ Social engineering
At a moment when politics in the US is at a standstill, the FED, unilaterally, is going to print 1T of money, without anyone's supervision, in order to restart the economy. Of course, the tax debate is irrelevant since this is brand new money out of thin air. The fact is that modern policy is conducted by the FED. It is also targeted at the haves, and will hurt the have-nots.
QE2, at heart is a way for the US to default on its foreign obligations. The risk is a debt trap, where the interest on debt consumes GDP. This is a way to solve the foreign debt trap problem. If the US wanted to get back at China for currency manipulation, this is it.
In general, QE2 is just creating a massive carry trade where, people borrow in dollars and just spend it OUTSIDE the US. Meaning the growth will be felt elsewhere. This is rooted in the fact that few growth opportunities are perceived to be left in the US and the differential with emerging markets is too high. The net result of this may be curtailed by capital controls being put in place.
4/ Trickle down QE
QE2 is printing money to buy assets (preferably bonds). This creates speculation in assets. If you are a asset holder, let's say a house, then the price of your house will artificially increase. This makes housing harder for those that don't have it, the young most notably. The basis for this as acknowledged by Bernanke, is that it will kickstart spending by the wealth effect. This is pure trickle down economics, conducted by the FED. Theory is give to the rich, at the expense of the poor, hoping it will all trickle down quickly.
5/ pushing on a string
Many commentators have pointed out that adding to bank reserves is unlikely to spur lending. Lending is driven by perceived opportunities, and there aren't many perceived in the domestic US.