SF is dirty in the gross parts, but it's not as dirty as places like Chicago, NYC or even Paris. It just so happens the dirtiest parts of SF are where startup people work and hang out: SOMA, Tenderloin, the Mission. The level of dirtiness in these areas is so off the scale (one regularly steps over human poo) that it warps one's perspective as to how dirty the city is in general. In contrast there is a layer of garbage and grime everywhere in New York City. So much so that after a day of walking around in the summer, you can turn a white handtowel black just by wiping the sweat off your brow...