Last night was interesting. As soon as I walked in the door from work I was accosted by my wife and a friend of ours, their mission: The Deadmines. They had made alts and leveled them up to 11 and 12 respectively. A holy priest and a fire mage. I have run instances all the way up to BRD solo and I'm pretty good at AoE farming. So what did I do? I put my mage in full damage mode and ran them though The Deadmines at least 4 times. In that whole time they went from 11 and 12 to 14 and 15. It didn't take long because my mage can kill most mobs in the instance with one blast, burn, explosion, or even a staff hit.
So with that in mind, how do you power level? I've found that pushing the characters through an instance with one 70 mage takes less time than running around gathering quests and getting them complete UNLESS the quests are in the area anyway.