Blizzard has released micro-adjustments to the Arena earlier today to help improve class balance. Mage and Warlocks received no changes while Hunter and Rogue received nerfs.

Do note that these changes should be somewhat shortlived as the Arena will be receiving changes in Patch 15.4 (date unknown) that remove the arena bucket system.

Quote From Dylan Bates

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.