I've taken a propeller to the face from a motor similar in strength to the consumer "drones", and it was spinning full throttle. I really don't recommend this. I got lucky it didn't hit my eye (came close). End result was some 4 scratches across the cheek on the left side but no permanent scarring.

In my case it didn't occur when I was flying the quadcopter but rather when I was bench testing the ESCs (yes, I knew abstractly I should remove the props before doing this but I got lazy after a while of testing and eventually it bit me).

Having said all of this I mostly agree with the grandparent post (though I do recognize that a 2 lbs drone could, theoretically, kill someone) in that the solution to this as a safety issue is primarily just don't fly a quadcopter over anything you wouldn't feel comfortable crashing it into (basically don't fly over people or valuable property it could potentially damage). As long as you stick by this rule of not flying over anything you're not afraid to crash it into you already have a good (if not inexpensive) failsafe, which is just cut the throttle completely.

Of course not, where did you get that idea? I don't think drones should ever be operated in close vicinity to people, especially if those people are bystanders uninvolved in the operation of said drone. Well yes, but the slight elevation of the sidewalk isn't the sole reason drivers don't run over pedestrians. The barriers are hopefully in the discipline of the pilot.