I think the single biggest problem with 3D printing is that most people don't have any idea what they would use it for. It's a neat concept, and it does seem useful that you could create a custom-made little plastic doodad of any specifications you want. The idea of being able to share designs seems to also have potential. Still, if someone gave me a 3D printer for free, I can't think of what I would use it for.
Maybe I just don't have enough imagination, but I think most of the population probably has even less than I do. There are only so many little plastic pieces of junk I need in my life. I think I'd get more use out of an automated loom that could make clothes, or an automated printer/binder that could make books. Or a system that made custom Ikea pieces for assembling custom furniture. I suppose you could make plastic furniture with a big enough 3D printer, but I don't want plastic furniture-- or a big enough 3D printer for that.
I've read through articles online about all the useful things you could make with your 3D printer. It's always stuff like book ends or door stops. Basically stuff that I don't really need, but if I did, the same purpose could be served by a small rock.