In 2005, New York City, with a population of approximately 8 million, had 539 murders. Washington, DC, with a population of 600,000, had 195.
There are a lot of explanations one could offer for this, but one of the best ones is the prevalence in New York of beat cops. DC actually has more cops per citizen--one for every 153 citizens, versus one for every 210 in New York. But after almost a year in DC, I've still never seen a cop walking on the street. I see them frequently-ish in their patrol cars, but almost never walking around among the population.
To be fair, DC is less dense, so it's harder to patrol than New York--but New York had patrolmen long before it had skyscrapers. And increasing the number of police on the street is among the most effective ways to reduce crime--unsurprising, since even people with very poor impulse control are able to keep from committing crimes when there's a policeman standing right there.
The good news is, DC may be changing its policy. In the wake of a particularly bloody weekend, DCist reports that the city is putting more cops on the beat. The local blog spotted a beat cop a mere three blocks from my house -- on a weekday.