Where Each City's Uninsured Live
Insured people are all alike, but uninsured people are uninsured for their own reasons.

It can be tempting to think of "the uninsured" as the poorest of the poor. But that's not entirely the case. While people living below the poverty line are the most likely to be without health insurance, 28 percent of people who make between 100 and 200 percent of poverty level (up to about $23,340) lack coverage, as do 15 percent of those who make between 200 and 400 percent (up to about $46,700).
These maps, created by Kevin Johnson and used here with permission, show where people not covered by either private or public insurance live in each city. Johnson used the 2012 American Community Survey; higher uninsured rates are represented by red and orange colors.
Some of the maps do show a clear rich/poor divide, like this one of Oakland and San Francisco:

But in other areas, its neither the wealthiest nor poorest communities that have the highest uninsured rates. In 27 states, Medicaid covers adults at the lowest end of the economic spectrum, even if they don't have dependent children. So in many areas, the uninsured are those who are too rich for public assistance but aren't connected to coverage through work for whatever reason. (This is perhaps best represented in the Obama administration's recent beseeching #GeeksGetCovered so they can launch more startups.)
These uncovered workers are spread throughout urban areas, representing freelancers, young people, and those who work part-time:

And you can see how local and state policies play a major role. Very few people in Boston, for example, are uninsured, because Massachusetts enacted its own healthcare law in 2006. The state's uninsured rate is the lowest in the country, at just 4 percent.

Meanwhile, Dallas has large swathes of uninsured people because Texas does not offer Medicaid coverage to low-income, able-bodied adults. It is also one of the two dozen states that opted not to expand Medicaid under Obamacare, so this problem isn't going away anytime soon. The state also has the highest uninsured rate in the country:

Nationally, blue-collar workers, the self-employed, and those who work for small businesses are most likely to be uninsured. The Affordable Care Act included a requirement that employers with between 50 and 100 workers offer health coverage, but that mandate has been delayed.
Some of these maps clearly show how work status influences insurance coverage. In Los Angeles, for example, there's a splash of orange on the city's east side, where there's a large immigrant community and throngs of young freelancers and aspiring artists.

Insurance rates can also signal 9-to-5 job status. Almost all of the uninsured on the Washington, D.C. map live outside the district's borders, near less-expensive housing and further from insurance-providing jobs. (Some commenters have also pointed out that D.C. has higher-than-average Medicaid cutoff levels and health insurance programs for lower-income adults, which is also a factor here):

A McKinsey and Co. survey found that only about 27 percent of Obamacare enrollees so far were previously uninsured. The Obama administration is entering its final push to urge the uncovered to sign up for Obamacare before the March 31 deadline, but that's shaping up to be a tough task. And that could be because each uninsured person has a unique job situation, financial status, and state legislative picture behind him. To destroy a perfectly good literary reference with dorkiness: Insured people are all alike, but uninsured people are uninsured for their own reasons. And that's going to make it hard to get everyone to want—and to be able to afford—a health plan.