What are the limitations of 3D printing technologies today and in the future?

Currently we can’t do anything with complicated insides, or anything made out of more than one material.

Most 3d printers also use only soft materials, though there are some that can deposit metal.

As far as i know, moving parts are quite a challenge still, however somewhat possible, there are a few videos on youtube about it. interesting stuff, not sure if its worth 5k investment.

One of the most interesting things i have seen, is the idea of using them to print body parts for medical use. but im sure we are still some way off for that.

The technology to produce a machined smooth bearing surface is still under development. Intersecting lasers in polymers, or metal deposition still leave a tiny bit that needs to be machined off manually to make perfect moving parts. and as @jackm says complicated innards are still a ways off. We will get there though.

There was just a NYTimes article about this.

Overhangs are difficult, but I think we’ll see a new revolution in manufacturing—especially small, individual builds—happening in the near future.

In fact, I’m building a makerbot in the next few weeks.