@parados,
The fall in the labor force participation rate (which was in decline well before Obama took office) can in large part be ascribed to two factors:
(1) The aging and retirement of the Baby Boom generation. The first members of that generation were born at the end of the Second World War and the last were born in 1964. From 1945 to 2010 is 65 years. Of course, not everyone waits until 65 to retire.
(2) Increased high-school graduation rates among minorities, particularly among the growing (and much younger on average) Hispanic population. Kids who stay in school don't necessarily join the labor force.
We can ask about "discouraged" workers, but with limited unemployment insurance they can only stay discouraged for so long, unless they are old enough to take early retirement on a pension or on Social Security; which means that discouraged workers can't account for long-term trends of decline in the labor force, except on the margins. (I'm willing to entertain counterarguments, however.)
I suspect that many public sector workers, such as schoolteachers, took early retirement options offered by the states during the Great Recession.
Incidentally, unlike either liberals or conservatives, I ascribe the long decrease in national crime rates (both violent and property crimes) since the early 1990s, to the same demographic change (the aging of the Baby Boom generation).
The first boomers reached adulthood in 1963, which was about when the long increase in the national crime rate began. By 1993 when crime rates began their long decline, the first boomers were nearly 50 years old and the last boomers were about to turn 30. Most crime is committed by the young.