[eschaton]

I mean, to my mind the fundamental disjuncture - the beginning of suburbia - was the electric streetcar because that was when you really started seeing a separation of the uses and the classes.

Earlier neighborhoods - even "proto-suburbs" connected to the core via horsecar, cable car, via railroads, or ferries - were all "walking neighborhoods." They were meant to be interacted with primarily on foot. Blocks were short, streets were narrow, and buildings on a given block could vary dramatically in terms of style, scope, and utilization. This was a necessity because every neighborhood needed to have residences available for all classes, every neighborhood needed commercial storefronts, and virtually all ended up having some industrial businesses as well. What this meant was a great deal of heterogeneity on each block, which makes them a pleasure to walk to just in the sense that you never knew what you were going to see when you turned the corner.

The electric streetcar changed this because it created the first neighborhoods not primarily meant to be dealt with on foot. Thus you got large swathes of land made up of interchangeable residential housing - often all made for people of basically identical socio-economic status - with nothing else nearby other than perhaps a tiny smattering of commercial. But really, not that much to see - or to walk to - because you aren't supposed to walk, you're supposed to catch the streetcar and go somewhere more interesting.