A Washington Post article discussing the risks associated with another round of quantitative easing raised the possibility that the Fed could lose its credibility if the program does not lead to the intended growth. It implies that the loss of credibility would be a major harm.
It is worth noting that the whole economic collapse came about because of the Fed's failure to notice and/or do anything about an $8 trillion housing bubble. Given this enormous failure, it is not clear how much credibility it currently enjoys among people who follow the economy.
The article also raises the risk that a precipitous fall in the dollar, "could be disastrous." It is difficult to see a scenario in which even the steepest falls in the dollar would be disastrous for the United States. U.S. exporters would suddenly become hyper-competitive (we still export $1 trillion a year in goods and services), while domestically produced goods would drive imports from the shelves. This scenario would likely be disastrous for our trading partners, which is why they would almost certainly intervene in currency markets to prevent the dollar from having a steep and sudden tumble.