The two lowest power/current ways to kill a human are defibrillation and brain damage, and 100mA AC is roughly the amount of current that is likely to stop a heart. This amount is specced in current rather than wattage because the lethal current would have different wattages and voltages with different paths of travel across the body. The wattage itself is still a danger in that when you lose muscular control you can't separate yourself from the circuit, so there are cases where people die by cooking. The wattage necessary to do that is rarely/never mentioned because it would depend on how long you were locked on and wouldn't really be a useful figure. Another example of lethal danger from wattage would be flash or explosion. An electrician, for example, doesn't need to know exactly how much wattage cooks him how fast. He needs to know he should shut off the systems he's working on when he can and use all available precautions when he can't. He uses insulated boots and a fibreglass ladder and work procedures that will minimise the chances of getting shocked at all.
Any shock that can kill you is a shock worth preventing. So if you take some safety courses, you may end up watching videos of electricity hurting or killing people in a variety of ways, but it would be rare to discuss these events by wattage in basic electrical safety when anything above the 50-100mA that can kill you is a no-no anyway.
On the other hand you may be interested in looking at arc flash suits. Because the suits are rated by how large of an arc flash (conducting fireball) you can survive, they are rated in categories and have calorie ratings required for each level of protection. The evaluation of how much danger is present in a system would involve wattage among other things, but is complex enough that it's handled by professionals.
Note also that the output of whatever is driving a load can fluctuate when the load is changed. Your 10V 1A motor circuit won't be able to pass enough current through you to more than make you tingle. Your 1000V 10mA drive circuit on the other hand could produce an initial high current surge even if it was current limited, and if it isn't current limited, its actual output will be determined by your body's impedance and the source impedance. If it drops to 50mA AC at 200VAC or 100mA at 100VAC, that could still kill you even though it represents the same wattage and is intended for 10mA output.