In theory, it is possible to not receive a shock at all when touching a live mains wire, even if you are standing in a pool of water. You just have to touch the wire fleetingly when it's at the zero point of its cycle. At the moment it's only a theory.
I think its because, with your hands holding the wires tightly, you're allowing a steady amount of current to flow that dissipates the energy. Without this steady discharge of energy, it builds up with the end result a high voltage and a high amount of energy to discharge in a short period of time. Hence the "shock".
Correct the old wind up mega would only produce 500 volt if open circuit, it had a limited amount of energy, so gripping hard likely under 50 volt, with a shock from mains you will not use enough energy to cause a volt drop. This is only correct while the neutral is connected, the problem arrives when the neutral is disconnected but the line is still connected which should not really happen, the neutral should only be switched with two pole switching, but borrowed neutrals do happen from time to time and when they do the neutral can become live when disconnected, this is one reason I like the neon screwdriver, today with clamp on ammeters there is a way to test before disconnecting, but years ago the neon screwdriver was the only way to get early warning when there was a borrowed neutral. This is why borrowed neutrals are outlawed, but it still happens.
Worst belt I've had was from the undone neutral of circuit with a whole load of florries on it, which I (obviously) thought wasn't energised. I'd put that down in my mind to there being a big live load there. Other shocks I've had when loads are off from similar circuits haven't tickled half as much. I guess a lot is down to how hard you grab hold of the bugger...
According to Bodge, the harder you grab the lighter the shock. (Only kidding - I know that Bodge was referring to a weird Meggery type device...)
It is a requirement in the UK and most other countries that the neutral is connected to earth at the supply transformer. Additionally, with TNC systems with a combined neutral and earth conductor (CNE), the CNE conductor must be connected to earth at various points along it's length, this is where the name Protective Multiple Earthing (PME) comes from. It is normal, when the system is loaded, to find a small voltage between earth and neutral. BS 7671 consider the neutral as a 'Live' conductor as it carries current in normal operation.
What is wrong with touching a live conductor? According to the 1953 7th edition of the American Electricians handbook, para 154 suggests contact with fingers at up to 250v to determine which in line and which neutral... and in para 155 use of the tongue for "low voltages" Would I follow their advice - unlikely!