What is Fed Chair Janet Yellen still so worried about that she feels interest rates must remain so low for so long?

A sign hangs in the window of a clothing store on June 6, 2014 in San Francisco, California.

The unemployment rate has come down from the crisis high of 10.0 percent (October 2009) to the current rate of 6.2 percent. We have generated an average of 230,000 jobs a month so far in 2014. Consumer sentiment has improved dramatically, the stock market is way up, the housing market has rebounded, and banks are finally beginning to scale up their lending activities again. What's not to like?