Forget DH, Hunter, Mage and Rogue. Judging by the new cards and streams so far, I’m going to go out there and predict Druid and Paladin cards are going to be the first to take nerfs once the expansion releases. Just because of their respective ability to take you out with gigantic minions sometimes before turn 7.
Druid: Most likely - Forest Warden Omu (6 - 7 mana)
Decent chance: Guardian Animals, lightning bloom (the dual class factor may help avoid the nerf however)
Paladin: Most likely - High Abbess Alura (4 - 5 mana)
Decent chance: Blessing of Authority (5 - 6 mana)