Nikabrik is at lvl 16, a dwarf hunter. He was in Loch Modan, but has hopped over to Westfall to get another level or two before finishing up the Loch Modan quests. And to get into Redridge. Unless someone asks him to group, I think we'll skip Deadmines for now.
Mrs Rosen, our lvl 21 human warlock, has a full quest log for Redridge and Duskwood, plus a few in the Wetlands. She's done everything in the Westfall and Loch Modan, except Deadmines.
Warrstand is at lvl 14 in Darkshore. We've only just started there.
But our fastest moving new character is Naattuu. She's flying along through the Barrens.
Ah, the Barrens.
As Horde, I typically get to the Barrens at about level 13 1/2 with 3-4 gold in my pocket, an adequate weapon and a mix of quest reward whites and cheap AH greens. Do the delivery to Orgrimmar or Thunder Bluff, run a quick errand down to Rachet, killing a few striders on the way, and ding — the character hits 14. While in Rachet, pick up more quests. There always plenty of quests at level or just below. I have yet to finish all the Barrens quests before they start going green on me. I could write my own Barrens guide. Maybe not the fastest, but certainly fun, easy, and efficient.