So far with my 4 shamans, I've only done quests. I skip most collection quests unless the quest item has a very high drop rate.

Quests that you want to do involve:
Kill Mobs X (named or otherwise)
Talk/Deliver to NPC
Escort
Use Item on mob/target

Less Desirable (depends on how many you boxing)
Pickup X Number of Objects (as long as its not too much)
Drop Quests (if the mob has a 50% drop rate or higher)

So far using this method, I have 4 level 20s at 14 Hours 20 mins playtime (including mod / multibox setup). My usual level time for level 20 is 11 Hours 45 Minutes so not big of a difference considering I'm gaining the same XP times 4 .

Though most this relies on prior knowledge of where to level and what quests to do in what order. I've leveled numerous characters to 70 (solo) so I have a good system / quest order that I follow.

You could also just have a 70 powerlevel you until Outlands but where is the fun in that?