There's a fairly persuasive article on the virtues of new urbanism over at Public Discourse.
I tend to be rather sympathetic to the aims of promoters of new-urbanism. People should live in communities, and towns should be built in a way that promotes that. We can quibble over whether this element or that is really necessary for the endeavor. I also find myself somewhat conflicted: I enjoy large trees and open expanses too much to be really comfortable in the cramped zero-lot-line environments that seem to be the product of much new-urbanist design.
But there's a more fundamental problem with the idea that if we just build mixed-use developments close together, that we'll automatically have functioning communities and be able to cooperate in shared spaces. That problem is, in essence, anti-discrimination law. The sort of idyllic cooperative that the new urbanists believe should be promoted through their architecture is only possible if the group of people living in a place is sufficiently homogeneous to operate as a society. You have to be able to exclude sociopaths, tattoo parlors, hookah lounges, wastrels, and Protestants from your community. Otherwise the cohesiveness necessary for the whole endeavor to function will never develop. We have proven quite incapable of excluding sociopaths from our existing public spaces over the last forty years: I fail to see how eliminating the front yard will banish them now. And federal and state law have eliminated most other means by which we might seek to control the demography of our local societies.
The new urbanists will have to show how implementation of their ideas will both reduce anti-social behaviors (vandalism, kidnapping, etc.) that afflict existing public spaces, and reach the desired results in the face of vigorously enforced anti-subsidiarity public policies.