Why are some AC outlets and plugs polarized?
My thought is that if the current is alternating that the hot and neutral effectively switch roles every 60th of a second.
The currents through both pins are equal and opposite, but the voltages on each pin are not. The neutral is roughly 0 V relative to the Earth at all times. The hot alternates between positive and negative.
It's all to do with how the power is transformed from high tension to normal mains voltages.
The power is transmitted across the country in 3 phases. There are two types of 3-phase electricity - star and delta. Delta uses 3 cables, and the power is split between them 120 degrees out of phase. When it comes to your local substation the power is fed through a transformer which switches it to "star" 3-phase. In this arrangement there are 3 live (or hot) wires and 1 neutral. The 3 coils of the transformer are linked together at this neutral connection, and, in relation to the other three lines, it has zero power. It is neutral. Very often this is also linked to earth at this point as well.
Star:
Delta:
As far as plugs go, if the power is being fed into a transformer internally to reduce the voltage, then the connections really don't matter which way around they go.
Some systems will employ switches and fuses and such in the input power, and this is best handled by the live connection, not the neutral, so a polarized plug helps ensure this. There are also grounding and 'commoning' issues to be taken into consideration.
The neutral and ground lines are tied together in your load center, and both are connected to a big copper stake in the ground. The only difference is that the neutral line is intended to have current flowing through it, while the ground line is not supposed to have current flowing through it.
While interrupting a neutral line with current flowing through it would expose you to high voltage, there's little to no potential on a properly functioning neutral line between the wire and the ground, so you can't be shocked.
Research GFIC (Ground Fault Interruption Circuit) sockets to learn about how this property has been applied.