I would like to add that a shaman means that you get tremor totem but with a priest you get fear ward but tremor totem > fear ward. Fear ward is always nice to have on the tank or healer to make sure you wont get feared in a critical situation.

My sugestion is to take a ele shaman over the mage or warlock.. I would sugest the mage btw!

Bloodlust/heroism can be a critical boost!
Tremor is making things easier in a few encounters. (the 3rd boss in Drak'thalon must be hell with a unlucky fear)
Fire,Nature,Frost resistance totems (im aware that the paladin brings Frost and Fire aura and they dont stack, but with a totem up you can go with devotion aure)
Totem of wrath