Also, tying back into what you said about efficiency: AC power can be much more efficient due to this easy transformation. The power dissipated across a resistor (which is a reasonably good model for power lines) is directly proportional to the square of the current passing through it. Thus, by stepping power up to high voltages, you can drastically cut the current and hence resistive losses in the line.
...but this whole thread was prefaced on room temperature superconductors. Hypothetically at least, this high voltage advantage wouldn't matter if you could have 0 transmission losses with DC at low voltages.
