Bar none, the worst electrical shock I've ever got, worse than 4160V AC even, was from the main storage battery on a submarine. Sure it's only DC, but it's 250+V and has the potential for over 10k Amps. I have scars from the third degree burns I can show people if they want to see them.
The human body has quite a bit of resistance to electrical shock. Typical values of resistance from one hand to the other through the body range in the 200kohm+ range. Wet skin, alchohol, dehydration, being tired, and other things all tend to lower the body's resistance to current flow. In general, the most commonly used # for body resistance is that it's always over 300 ohms, and since the standard for what will stop the heart was traditionaly always 0.1A, a little math with ohms law leads to 30 volts being the safe limit for working with electricity. (This is 9/10 of the time the value of voltage that if exceded in the workplace the worker is required to use precautions to prevent shock, and usually the cut off that seperates "low voltage" systems from "high voltage" systems.)
All that said, most of the time on dry skin you won't feel a shock from 12VDC unless something else is going on. The # most often used for feeling a shock is .001A, and with only 12V to push it you would need a resistance less than 12kohms to feel it, which is rather unusual. Not impossible, but unusual.
Now, back to the discussion at hand, "ground" is an arbitrary point in any electrical system. When you establish ground in a system you are establishing the point with which you intend to reference everything. It does not necessarilly reflect anything with respect to the dirt under your feet. Voltage is a potential energy until the current actually starts to flow, so it's a lot like gravity. If you pour water from a cup into the sink, and the sink is 12" below the cup, the water will hit with a certain ammount of force. That ammount of force is completely independant of your location when you pour the water, and will be equal in every location where you pour the water. Death Valley or Denver, the same splash in the sink. That 12" only has meaning when you set a reerence point, the sink, and measure from there. Voltage works the same way, only with respect to one terminal does the other terminal of a battery have any potential.
My Dodge truck runs 2 alternators, one with the output +12V with respect to the chassis and the other -12V with respect to the chassis. The 2 batteries are then wired so one has it's (+) terminal grounded and the other has it's (-) terminal grounded. All my 12V acessories work fine, as long as they are hooked up from one battery's (+) to the same battery's (-), but if I need 24V for something, like the really big winch I got military surplus really cheap, I can hook it from one battery to the other and get 24V.
Oh, and the starter worked because it's a series wound DC motor, which means that even though the field on the rotor reversed magnetic poles due to the reversed voltage so did the field in the stator, so the effect on rotation was the same. If both south poles turn into north poles at the same time they will still repulse each other the same as when they were magnetic souths.
From the bleacher seats, anyway...