Already mentioned: danger to humans.

Also: telephone lines, trees, birds.

Over at MAKE, Eric Weinhoffer adds a few more: battery limitations, weather, theft. CNN says: "The sensor technology to avoid collisions isn't there yet." Mother Jones points out: Some states have banned drone cameras (which Amazon would need to make this feasible). And Bezos's own Washington Post warns: Hackers.

Once all that's solved, there's the FAA. Good luck, Mr. Bezos.