Grinding mobs isn't the best way to gain XP, questing is.

I have noticed you FLY thru levels until about level 12-14 (basically, when you venture outside your noob areas). From that point on, I've switched to a new character each time. Right now, I have a 14 Shaman/13 Warrior/6 Warlock.

Like I stated elsewhere, I'm still getting a feel for each class/tradeskill.