First, location is the overriding key to value and that can change over time for better or worse. Business closings or relocations (i.e., the economic lifeline gets cut or reduced), deteriorating neighborhoods, etc., have more effect on price than any structural components. The trouble is that if you’re talking about your personal residence, then these conditions provide motivation to sell and move at a time when it becomes expensive to do so.
@JasonR’s post on modern conveniences reminded me of a quote, I think it was Mark Twain, saying he would gladly exchange the entire Acropolis (a classic if ever there was one) for one good American bathroom.
And another Frank Lloyd Wright classic, Falling Waters, has had its problems:
Given the humid environment directly over running water, mold had proven a problem. The elder Kaufmann called Fallingwater "a seven-bucket building" for its leaks, and nicknamed it "Rising Mildew". Condensation under roofing membranes was also an issue, due to the lack of damp proofing or thermal breaks.
Fallingwater's structural system includes a series of very bold reinforced concrete cantilevered balconies; however, the house had problems from the beginning. Pronounced deflection of the concrete cantilevers was noticed as soon as formwork was removed at the construction stage. This deflection continued to increase over time, and eventually reached 7 inches (over a 15-foot span).
In 1995, the Western Pennsylvania Conservancy commissioned a study of Fallingwater’s structural integrity. Structural engineers analyzed the movement of the cantilevers over time and conducted radar studies of the cantilevers to locate and quantify the reinforcement. These showed that the contractor had indeed added reinforcement over Wright's plan; nevertheless, the cantilevers were still insufficiently reinforced. In fact, both the concrete and its steel reinforcement were shockingly close to their failure limits. As a result, in 1997, temporary girders were installed beneath the cantilevers to carry their weight.
When it comes to structure though, classics, especially those determined by then-popular taste, is no substitute for knowing the difference between good construction and poor. And it can sometimes go against modern construction. For example, in NYC, “pre-war” is a selling point because the construction was more evenly good back then, as compared with post-war shortcuts. Even now, some luxury buildings with units selling in the millions (one sign advertised the units as being “worth every million”) have all the luxury trimmings but poor underlying construction. Some are built using 10” concrete pads, which will last maybe 10 years before some really expensive and intrusive shoring and repair work will be needed. But they have $25,000 Viking refrigerators!