I agree that the larger culture suffers as people retreat from it, and no more is that more evident that the city. A large city like Los Angeles feels much more healthy culturally than a small city like San Francisco, and I think it's because people don't feel the need to retreat into their own subcultures, and not give anything back to the larger community. What makes Los Angeles great is the ability for many of the people to communicate between subcultures, and that would be my argument for what gives a place a "healthy" culture.
However, the American culture as a whole has always been fragmented, and perhaps it isn't possible to reconcile the separate parts. I certainly tried to cross between Los Angeles, Chicago, Portland, and San Francisco cultures, and found that it is almost impossible to understand the deep cultural assumptions of people who identify with one city, given experience with another.