Knowledge of mechanism arrangement, tolerance stack up, motion kinematics, material properties, and fabrication methods are just not part of everyone's typical abilities. But the 3D printer addresses one of the big elements -- fabrication -- by enabling a person to quite literally click a mouse button and build a complex object.
I assume the term killer app came from the phrase "to make a killing." I never thought much about the metaphor -- the use of the word "killing" as a synonym for "succeeding" or "winning." But it's a little creepy to think about what the word killer really means. After all, a "murder app" or "homicidal app" doesn't sound appealing, and yet all these words are interchangeable synonyms.
My husband and I used to make fun of the couples in restaurants who stared longingly at their phones rather than each other. Over plates of over-priced food, they gazed at their screens while never once making eye contact. We vowed never to be that couple. Then, we each got a smartphone -- on the same day.