We’ve just released an update to all regions in order to adjust the appearance rate of cards to ensure a fair balance between classes in Arenas.
Specifically, the win-rate of Hunter and Rogue should now be decreased. The win-rate of Druid, Paladin, Priest, Shaman and Warrior should now be increased. Card appearance rates for Mage and Warlock remain untouched.
The current card buckets can be viewed on the Arena Rules thread here.