If you have a few minutes, Time has a good article here about the drop in crime over the last several years -- and the failure to explain exactly what's causing it. They go over a few theories, including a few that I hadn't heard before. Some experts believe that we're experiencing a drop in a crime because, on average, the U.S. population is older, and most criminals tend to be young.
Others credit a change in the drug trade. Heroin and marijuana are more popular than crack now -- which means drug users now tend to be docile and quiet, not angry and violent.