In a post about teaching quality Matt Yglesias from CAP discusses military personnel quality: 

In the aftermath of the Vietnam War there was a very low level of interest in joining the United States military and consequently in order to maintain the required overall force size it was necessary to make recruiting standards quite low and to be pretty lax about who you would keep on. The rebuilding of the quality of the personnel employed by the military over the course of the 1980s and 1990s is one of the great prides of the officers who were involved. And when the Iraq War was leading to a personnel crunch and moves toward diluting recruiting standards there was, rightly, a great deal of hand-wringing over it. Concern about personnel quality is also one of, if not the, main reason why the military brass is generally very hostile to the idea of conscription and this is also why we’ve encouraged our NATO allies to abolish conscript and build smaller, higher-quality, more professionalized forces.