Having lived in other large US cities, I am always astounded by the number, brazenness, and aggressiveness of SF's homeless population.
From my (anecdotal) observations, poor police enforcement appears to be a motivator for homeless people to gather and remain in SF. Walk a couple laps around City Hall and count the number of misdemeanors you can spot in the open and homeless 'tents'. Most other cities don't allow anything of the sort.
