With all the debating we could do about what metal means, let's look at it critically as sound. It's a violent, powerful, uncentered sound. Anything can happen. When you listen to death metal or even heavy metal, you expect sudden riff changes (nature of power chord riffs as harmonically movable) and there's a constant bassy explosive distortion sound. Even Black Sabbath sounded like a war machine cut loose across the land compared to the (limp-wristed leftist hippie) music of the day.
I'd say rock is moralistic. It starts with a pleasant introduction, deepens into a droning riff or two, and operates around a clear tonal center with ambiguous pentatonic riffing that easily changes key without expressing much except rhythm. It's simpleton music.
Power is what metal conveys. Power is a death of morality. Power is a sense of life being important, like riding a wave or conquering an enemy, and inherently refutes the kind of moralistic crowd-pleasing pap that rock music is. I think that's why we listen to heavy metal and why we love it.