Via Kottke, here are black-and-white photos of 16 locations in New York from the mid-1930s, and then again (from the same angle) in the mid-1990s. Some have changed utterly, others hardly at all. The city has become taller, less messy, less industrial, and busier.
New York is iconic, so most people recognize these places, but you could make similar comparison in any city. The differences would be even more stark here in Vancouver. I think of a photo my dad found in his workshop a few weeks ago. He immigrated from Germany in 1955, and took the picture from a stone pedestrian overpass in Stanley Park in '57 or '58. It looks back on downtown Vancouver, with the only tall buildings being the Hotel Vancouver, a new apartment complex on Georgia Street, and the B.C. Hydro office tower—then under construction, and now converted to condominiums. From that same spot today (the overpass remains), you can't see any of those structures, because a forest of hundreds of glass-walled skyscrapers intervenes.
In fact, here, go look at the differences in Vancouver in a mere 25 years between 1978 and 2003.