Michael Chricton was on Charlies Rose a few years back, and his biggest worry for humanity was something along the lines of self-replicating nano-bots. Without going into specifics, he said in 20-30 years we'd be at the point where we would need to decide as humans whether this tech was indeed safe enough to use.
He also stated that now, for the first time in his life he talked to scientists who were deciding not to pursue a line of science due to the potential risk to humanity.
