The U.S. job market is slowly improving, and most economists expect that gradual recovery to continue this year. Yet one of the most disturbing trends of the Great Recession is still very far from being reversed. America's middle-class jobs have been decimated since 2007, replaced largely by low-wage jobs.

A recent presentation from the Federal Reserve Bank of San Francisco lays out the situation clearly, citing data from the National Employment Law Project: The vast majority of job losses during the recession — about 60 percent — were in middle-income occupations paying between $13.83 and $21.13 per hour. But those mid-wage jobs have made up just 27 percent of the jobs gained during the recovery.

By contrast, low-wage occupations paying less than $13.83 per hour have utterly dominated the recovery, with 58 percent of the job gains since 2010.