The purpose of a subway map

What should a subway map tell us? The old NYC abstract map is being updated for a magazine. It was criticized for being to uninformative, though the London Tube map is generally praised for it's simplicity.
As we learn more about how people navigate their cities, whether by nodes, destinations, landmarks or street signs, we might gain more insight as to what a map should include. It seems that with fixed and infrequent stations-like a subway-a simple map is a nice compliment to a more detailed map that becomes redundant as locals gain knowledge of their city.