Balance is crucial in a game like Hearthstone, and with each expansion introducing 135 cards into the game, that delicate balance can get easily disrupted. A re-balancing act is usually necessary, and that's precisely what Blizzard is implementing today, nerfing four specific cards.
First up is the Mage's Conjurer's Calling, going from 5 mana cost to 7 mana cost, making it harder for the Mage to dominate the board so soon in the match. Next is the Warrior's Dr. Boom, Mad Genius, going from 7 mana cost to 9 mana cost. This mean the card is coming later in the match, and not only that, it makes it a lot harder to use the hero power when it's played. Finally, there's the Priest's Extra Arms, getting a cost increase from 2 to 3 mana.
These three cards are all in the current standard set, but a wild card also got nerfed, Barnes. The oh so popular card got a cost increase from 4 to 5 mana, giving the opponent a better chance to react to it.
You can read more about Blizzard's reasoning for the nerfs here.
