According to Charles Jencks, the author of Iconic Building, . . . iconic buildings [are a] delicate balancing act between what he calls explicit signs and implicit symbols, that is, between an unusual, memorable form and the images it conjures up. He emphasizes that in an increasingly heterogeneous world, multiple and sometimes even enigmatic meanings are precisely what turn a building into a popular icon.
What Jencks does not say, in his altogether too polite book, is that this balancing act is extremely rare—for every successful icon there are scores of failures.
Well worth a quick read.