So the legendary Thomas Krens is stepping aside at the Guggenheim. “This departure is a genuine art world event, because of Mr. Krens’s influence and buccaneering style. He has done more to redefine museum practice than anyone since Thomas Hoving was director of the Metropolitan Museum of Art in the 1960s and 1970s. Still, as with Mr. Hoving, the surprise isn’t that his departure took place, but that it didn’t happen sooner.”