Arena Balance Update - Hotfix
Blizzard just announced that they are in a process of deploying a small server-side patch (you don't need to download anything) for Arena, which should ensure a more balanced environment for all classes.
Quote from Blizzard
We’re in the process of deploying a small server-side hotfix that will update the appearance rate of cards in Arena to ensure class balance remains close to the ideal 50% win-rate. Specifically, the win-rate of Demon Hunter, Paladin, Shaman, and Warlock should now be decreased. The win-rate of Druid, Hunter, Mage, Priest, and Rogue should now be increased. The win-rate of Warrior should be roughly the same as before.