Denver doesn't hold a candle to San Francisco. Yes, right here in left-coast city, the poor and the rich ARE on each other's doorsteps or, more precisely, the poor are on the doorsteps of the rich, the poor lacking doorsteps of their own.
I grew up in SF and manage to continue living here because I inherited the house. But I've watched as the divide between rich and poor has reached obscene levels. There are plenty of statistics to quantify the division. The Census has computed SF's Gini Index as > 0.5, the most extreme of any California county, and more extreme than most 3rd world countries. (https://en.wikipedia.org/wiki/List_of_countries_by_income_equality)
Even the stodgy SF Chronicle had an article that the middle class is now a minority in SF. In 2005-09 only 11% of San Francisco households earned $150,000 or more. In 2010-14 that had grown to 32%. This is a huge increase, even accounting for inflation. Meanwhile, the % living in poverty grew from 11.5% in 2009 to 13.3% in 2014.(http://www.sfchronicle.com/bayarea/article/S-F-s-vanishing-middle-class-Dramatic-growth-6679627.php?t=39b2d80ea3&cmpid=twitter-premium)
When I walk the streets, half the people are obliviously talking into their cell phones, the other half are obliviously talking to themselves. The clash between the truly privileged and the truly destitute is painful to anyone who hasn't lost their humanity. Fortunately for most San Franciscans, I think they already have.