Adding to the above post, which brings up an important point, the plethora of CSI-type television shows has reated an unrealistic portrait of law enforcement capabilities, especially at the local level, and de-emphasizes the citizen role in community policing and crime rates.
There are some problems that law enforcement has always faced, and will likely continue to do so. High crime rates, limited manpower, violent offenders, high stress and job burnout rates. But I think, especially in the last few years, some even larger challenges have emerged to make the job of law enforcement that much more difficult.
Almost every state in the country is in some degree of budgetary crisis. Billions have been cut from state spending, and to achieve those kinds of cuts means hurting every essential program a state provides, from basic education and health care to, of course, law enforcement. Like so many government agencies, law enforcement is being told to do more with less, and they are a public service that has often been underfunded.
On a national and local scale, we have some very high tech, well-financed, brutal and terroristic criminal gangs that have infiltrated the US borderlands and cities. Police at every level often find themselves outgunned by a determined criminal element that seems willing to do anything to maintain their illegal profits.
It's a noble career, but one has to enter it with eyes wide open as to the challenges and limitations people in that career face on a daily basis.