A bunch of good points are being brought up in the comments.
First to point to the differences between the tail and main rotors, despite being locked through the transmission and rotation in synchrony (except when pedals are depressed) the speed of rotation and angle of incidence between the two are quite different. The tail rotor is balanced to only counter the rotational effects of the main disk, not provide the lifting force for the helicopter. So any thoughts of an equal comparison between the two is not appropriate.
Now for the ideas about the main rotor system, there is certainly a difference in speed between the center of the rotor and the tips, with the tips traveling at a speed somewhere on the order of .6 mach. Depending on the position of the collective the angle of incidence of the main rotors is changed. A worst case scenario could be achieved through a horizontal blade angle at time of incident.
Having seen 2 helicopter flight helmets which were struck by the main rotor in my time as a flight medic I can say that the mechanism of injury is certainly severe enough to result in clean cuts to unprotected human anatomy. This is also in line with the actual fatalities that have occurred from contact with the rotor system.
The speed of the blade compensates for the perceived dullness of its leading edge. Try pressing your finger against a sharp knife, versus dragging it down one. If you dont believe that the edge is relevant (since its too sharp compared to a rotor) maybe try an axe, since they are honed to being roughly 2x as blunt as a knife. if you cant draw blood on the axe, move your finger quicker, im sure it will feel just like a razor blade, from personal experience.