The first key to thinking about 3-D printers is this: Do not think printer. Think magic box that creates any object you can imagine.
In the box, razor-thin layers of powdered material (acrylic, nylon, silver, whatever) pile one on top of the other, and then, voila — you've got a shoe, or a cup, or a ring, or an iPhone case.
It's miraculous to see. Press a button, make anything you want. But just how important is 3-D printing? Unlike earlier big-deal technologies (like, say, the tractor) 3-D printing won't really replace what came before.