As an addendum to the last post, Ryan Avent notes:

I don't mean to pick on Atrios, because he does a lot of great urbanism stuff, but this is also only half right. You can have walkability and a big yard, so long as you aren't picky about whether or not the yard is your own private property. That's the tradeoff.

The essence of a good, walkable, urban place is density, but it's density that's achieved in part by publicly supplying a lot of the things people want, and thereby achieving a more efficient use of space. Is it necessary for every last home on the block to have a decorative, manicured lawn that does nothing but sit there getting watered and mowed? No way, a couple of public gardens will suffice. Is it necessary for every home to have its own blacktop square with basketball hoop that sits unused 99 percent of the time? Is it necessary for every home to have a place for a father and son to play catch, or for a guy to sit in the shade and read?

A well-planned city will provide good public spaces-sidewalks and retail corridors and public gardens and parks. When you frame the tradeoff as being between open space and walkability, many will say, oh well, it would have been cool to walk places. But that's not necessarily how it has to be. Washingtonians can sit on their patio or balcony and grill or have a coffee, and when they need more space to shoot hoops or admire some greenery or whatever, there are plenty of places to do that. And when they don't want to make their own coffee and sit by themselves, well, they can walk to places where they don't have to.

Parks are definitely the key to building a city that works over the entire life-cycle.  But I'd dispute that DC has done a good job of this.  Indeed, this is one of the things that my mother, who has just moved here from New York, often complains about--there aren't really any adequate parks in Northwest, at least east of Rock Creek.  I live a few blocks from one of the better ones, but it's not very good for dogs, and no good for children at all; it's basically a place for adults to take a stroll (and at night, to sell drugs). 

Middle class families are, IMHO, the backbone of a thriving city--they're the stabilizing force that keeps civil society together.  And those families will not stay in DC, in part because of the schools, but also in part because DC is not constructed to make it easy to have small children here.  In New York, on the other hand, there are dozens and dozens of neighborhoods where families can live within walking distance of a sizeable park, replete with playground equipment, sledding hills, and fountains to splash in.