Most importantly, he frequently used the one word that's become anathema within the engineering culture where adding features is thought to be a positive, as well as in the broader culture where collaboration necessitates acquiescence to groupthink. That word was "No."
Saying no is the most effective way of keeping an engineering project within its design constraints, of reining in the cost of bills of materials, and of generally keeping schedules on track. Subjectively, I believe it also enforces an elegance of design that began to flourish in the arts with Bauhaus and Deco. These in turn led to streamlining, which reached its apex in the era of locomotives. They also begat Harley Earl, the legendary General Motors designer of the mid-twentieth century.
Collectively, those influences are reflected in the iPhone and iPad. They're also fusing into the miniaturized designs we're working on today. I believe miniaturization itself compels enticing packaging.
Petroski writes that engineers exploit scientific principles and discoveries. That's true, but I think the best definition of good engineering can't be captured by a dictionary. Rather, you just know it when you see it.