Walking Firenze
Leon Krier, from “The City Within the City”.:

THE QUARTERS.
A city can only be reconstructed in the form of urban quarters. A large or a small city can only be reorganized as a large or a small number of urban quarters; as a federation of autonomous quarters. Each quarter must have its own center, periphery and limit. Each quarter must be A CITY WITHIN A CITY. The quarter must integrate all daily functions of urban life (dwelling, working, leisure) within a territory dimensioned on the basis of the comfort of a walking person; not exceeding 35 hectares (80 acres) in surface and 15,000 inhabitants. Tiredness sets a natural limit to what a human being is prepared to walk daily and this limit has taught mankind all through history the size of rural or urban communities.

One of the attractions of my own city, Montpelier, is that I can walk across it comfortably in well under half a day — and can walk out of it and into the countryside in about 10 minutes, which is also how long it takes me to reach the city center. Despite that we have little mass transit, I rarely use a car.
Krier’s point about quarters — that they must integrate all daily functions of urbain life — seems spot on to me when I think of the cities (and parts of cities) that I’ve found most agreeable.
For more along these lines, check out James Howard Kuntsler’s website and Steven J. Dubner’s Freakonomic’s blog entry on the future of suburbia.