I TEACH a class on architecture and urbanism at the Wharton School of the University of Pennsylvania to real-estate majors and MBAs. Last semester, I invited the CEO of a national property-development company to talk to my class about issues affecting the design of shopping malls. At the end of his presentation, he offered the students two observations. “The wonderful thing about this business,” he said, “’is that if something works you can repeat it over and over. The problem is that you can’t take long vacations—things just change too quickly.”