Current that flows thru the body Damp, salty skin conducts better than dry.
The path, say, thru a nerve, the brain, or heart, rather than just surface skin.
Duration, time in seconds. 50-60 cycle AC has this fibrillation effect so it is hard to let go. DC causes One muscle contraction that tend s to make U jump away. This is why DC used in defibs. AC would make it worse.
Frequency. A hi frequency like 25KC will travel 90% on surface so as not to stop the heart.
Also, if wearing pacemaker, These are sensitive to any current flow or even induced by nearby RF transmitters.
EDIT: AC more Destructive than DC? Can be. The higher the AC frequency, the more the skin effect, where it is less destructive. Until you get to hi power RF and microwaves, which can boil your body's moisture like a microwave oven . Even at only 27 MHz, hi power can still cause incredible burns to the skin, even if it does not penetrate deeper. This can cause someone to fall and get killed. At 50/60 Hz * line frequency, 110 VAC (RMS) is equivalent to 110VDC in voltage supplied, so, same current and power to same (Resistive) load. Low frequency AC tends to "fibrillate" muscles in a rhythmic contraction/relaxation, like the heart. This is certainly "destructive" to life. And , causes arm muscles to fib, the: "cant let go" phenomenon. Increasing duration of shock and burns. DC tends to contract muscles, once, you fly off. Safer in This respect. This is why DC used on streetcars./subways, and for defibrillators. AC in a defib would only make things worse.
* aircraft often use 400 Hz, the shock effect and fibrillation is only slightly less than 50/60 cycle.
electronics tech 40 years. 1st class FCC and ham "extra" ticket, build Tesla coils and other hi-voltage devices, dad was electrician's mate USN 20 years.