safety
What's the point below which electrical current is generally considered safe for "casual" human contact?
Is either voltage or amperage more "dangerous" (e.g. high voltage / low amperage vs. low voltage / high amperage), or is the only consideration the total current?
How much voltage is dangerous is not really a static number as it depends on your body resistance, time of exposure and source "stiffness" (i.e. how much current it can supply). You get figures like 60V (or as low as 30V) which are an attempt at an average figure above which "caution should be taken".
However, depending on how "conductive" you are at any one time, sometimes e.g. 50V might be quite safe and other times it may kill you.
DC or AC (and what frequency) seem to make a difference too, female or male, etc - this table is very instructive:
Figures as low as 20mA across the heart are given as possibly capable of inducing fibrillation - here is another table from the same source that gives body resistance based on different situations:
You can see that as low as 20V may be dangerous given the right conditions.
Here is the reference the tables came from, I think it is quite accurate based on some experiments I have done myself measuring body resistances. The rest of the site seems to be generally very well informed and presented from the bits I have read, so I think this may be quite a trustworthy source.
Power out of wall-warts is solely determined by the load you connect to it and not by the potential power that can be fed from the AC wiring.
Say it's a battery charger and it charges 4 x 1.5V batteries at 100mA - power out of the charger = power into the batteries = 6V * 0.1A = 600mW.
AC power into the charger is 600mW plus some small residual power related to how efficient the wall-wart is. Typically 700mW to 1W.
Power output from 120Vac might be 20A * 120V to power a kettle = 2.4kW. It might be a lot more but it is the load or appliance that dictates power taken.
Things like this are made safe because wires are insulated and circuits are isolated. Output voltages are kept low so that the effect of electric shock is barely felt.
Same with current - current drawn by a circuit is as per the load circuit demands and there are protection devices that inhibit the current beyond the intended use of the wall-wart - not to protect the user (because the low voltage does that) but to stop the electronics inside frying when someone shorts the output.
Wall-warts usually have AC line fuses inside them to prevent faults devloping into fires.
Best Answer
As rules of thumb, I've generally thought of myself as a 70 kΩ resistor to ground that feels pain at around 1 mA, which can be driven by 70 V or so. In my experience, the pain threshold is slightly above 48 V.
I can't say that I have any good medical science to back this up, but there are a few empirically obtained data points in that I'm not dead yet.