[digerata]

The forth paragraph in that link really sums up the difference for my poor brain:

"One advantage of direct current over AC is that DC current penetrates the entire conductor as opposed to AC current which only penetrates to the so-called skin depth. For the same conductor size the effective resistance is greater with AC than DC, so that more power is lost as heat. In general the power losses for HVDC are less than an AC line if the line length is over 500 -600 miles and with advances in conversion technology this distance has been reduced considerably."

[deelowe]

Right. Additionally, frequency plays a role and AC can ionize the air at higher voltages, which increase impedance super-linearly. Basically, high voltage DC is probably going to become a very attractive option as high voltage transistors become commonplace. The reason AC was so attractive previously was b/c it's very easy to convert voltage to current and vice versa with just some coils of wire. With silicon based solutions, that's becoming a much less efficient (and even cost more costly) option.