Shamans don't start getting good till 32+ elemental wise, and lvl 30 for enhancement. Leveling shamans is a little bit slower till outlands. But over all after you get geared Shaman > Warlock a lot more burst dmg. Warlocks/Priest or Warlocks/druids = fun though. You have a mobile dot force ready to kite 34759347534 mobs. Hopefully with pet changes they will be able to do heroics.