Invisibly but utterly pervasively, penetrating every aspect the visible face of city, from the newest buildings in the oldest neighborhoods and from the grandest prospects to the litter on the sidewalks, there is another infrastructure, one of a purely social kind, the set of interlocking social facts which comprises the relationships of property and rents (incomes derived from real property).
So any serious answer to the question of what makes a neighborhood must at some point at least acknowledge the role that property and rent have to play in the ongoing neighborhood-constitutive process and, with it, the on-going processes of change in the visible aspects of a neighborhood.
And it would be fair to assert, though very difficult, I think, to demonstrate, that the visible changes, possibly all of them, are, among other things, signs of change in the relationships of property values to one another in the block, the neighborhood, the borough, the city, even the country and the world, and the interaction between property values and rents, and the flows of owners, renters, businesses, people — through those properties.
The tenants that can afford to move in and stay for some greater or lesser span of time and the tenants who can afford to leave voluntarily for something "better" or who for whatever reason can no longer afford to stay and so must, involuntarily, go somewhere else (or possibly, in the case of businesses, simply go out of business — which I suppose also happens to human tenants from time to time too, despair at having to move surely motivating, at least in part, some of the city's suicides as well some of its deaths due to malnutrition, disease, heat, cold, etc., that may in part, in some instances, result from preferring the otherwise unaffordable home to an adequate diet, health care, a livable range of temperatures, electricity, a telephone, etc.).
So ultimately the question would be: what do the visible signs of change tell us, however indirectly, about the invisible changes in the structure of property values in the city (and of course, by extension, the legal framework surrounding and supporting those property values), and their consequences for the "demographics" (in the broadest sense) of the flow people and their enterprises through these properties?
A tall order, that question, and with it the possibility that the distances between the signs of change and the underlying changes in property values are, for the most part, too great, too heavily mediated by, well, everything, for the visible world of the street to reflect much else but the largest scale changes over relatively long stretches of time.
Or maybe not, maybe one just has to learn how to read them.
My guess is that we have learned to read them, just by living long enough in the city, without exactly, necessarily, even knowing that we've done so, so that "learning to read" the signs of change is more a matter of making more explicit some of what we already know implicitly than it is a matter of learning something entirely new.
And what of it? What if we knew? What if we could read those signs? What then?
Kant famously introduced the concluding sections of his Kritik der reinen Vernunft (Critique of pure Reason) with the three questions: What can I know? What ought I do? What may I hope? These questions, Kant says, represent the three principal interests that motivate the different kinds of knowledge that are available to us.
And so we may ask: What can we know about the dynamics of the city? What ought we do about them? What may we hope for them — and for us?
If I knew, I'd tell you. So I think I'll leave it at that for now. But John Logan and Harvey Molotch's Urban Fortunes is a good place to start.