3D printing has already resulted in the creation of a firearm that cannot be detected by metal detectors. As 3D printers become more accessible and criminals get a hold of them it will change the world. Who knows what these ill minded people will come up with next. Whatever it is, it will change the world.
3D printing is, to be a total fanboy about it, really neat. I tend to think of it in terms of Star Trek and this new style of printing is the very first generation of what our ancestors will call a replicator. 3D printing makes building small parts faster and more economical for a variety of functions. I just read the other week that surgeons are experimenting with 3D printed bones for spinal correction surgery. Like I said, it's just neat!