Watching the latest Mythbuster episode is enlightening, particularly if you take the weights of the crafts they're testing into account.

THe TL;DR of the episode is - carbon fiber blades directly connected to motors (likely the setup for commercial UAVs) is capable of slicing deeply enough to hit your jugular, others are not (unless the motor is being held solidly and used as a form of saw).

I'm more concerned with UAVs and payloads in excess of 10 lbs falling from 400 feet up onto my head because a wiring connection came loose and cut power to the UAV (happened to me recently, minus the head).

In short, don't fly UAVs around people, or in no-fly zones.