I dont think that shammys are overpowered. Theyre just really funny to play with. If you are a skilled player, it shouldnt be a problem to beat other classes (I still cant kill that damn hunter).

Mage: I think theyre the easiest enemy to kill.
Priest: Not as easy as mage but still.. That earthshock is a really strong spell.
Warlock: Too bad if he succeeds with fear, and deals massive dmg with those DoTs.
Druid: Still quite easy kill for Enh shammy. Just dont let him heal and kill him with WF..
Paladin: This is the hardest kill I think. Most of them dont deal very high amount of dmg, but those battles take too long cause of their bubble. Stay far enough.
Warrior: Quite easy. Just dont let them get to you.
Hunter: Well.. This is very hard.. Im still trying different strategies, but the best is to get near him and beat him.
Rogue: With those stuns its best to run away and just slow him with grounding etc :)