Current flow is a function of voltage and resistance, as per ohm's law. The human body's resistance, even under worst case conditions, is almost always high enough that you won't even feel a 12-volt battery. You can't get 400 amps to flow through a 10,000 ohm resistor with 12 volts.
You're talking about whole different scenario when you talk about shorting the battery. You're unleashing the battery's stored energy in a short-term burst. I have a melted wrench from just such a thing - tightening the positive cable end while touching the car's body produced a pretty big spark and put quite a nick on both the wrench and the radiator core support structure. Though it did startle me a bit, I would hardly qualify it as torture. This is why, on a negative ground system as found in nearly every car, you should disconnect the negative first and re-connect it last.