While we still have to wait for Whispers of the Old Gods to live April 26, Blizzard gave us our first helping by announcing card adjustments to some of the original cards.
Blizzard has generally taken a light touch with making changes to existing cards (with the addition of Standard format), creating what is essentially a set of cards that doesn't cycle out. Getting some of the most powerful permanent Standard cards in check was important to keeping the meta dynamic. An overpowered "forever" card can make every new card be forced to fight against it for relevance, the same way Piloted Shredder casted a shadow over all new four drops.
ANCIENT OF LORE
THE NERF: Draw one card instead of drawing two cards.
Must-include cards, especially those that don't cycle out, create a difficult task for designers trying to introduce cards that change archetypes. When a card is so powerful that you nearly always include it, it ceases to become interesting. Deck-making ought to be a balance of benefits and trade-offs.
Ancient of Lore was a must-include in Druid decks simply because it was so good. It's one of only two minions in the game that draws two cards. The other card with the same ability, Coldlight Oracle, has the massive downside of drawing your opponents' two cards. If you go by rule of thumb that a card draw is worth roughly 1.5 mana, that leaves Ancient of Lore essentially a four-mana 5/5 outside the car draw.
The only other 5/5 cards costing fewer than five mana are King Mukla and Anub'ar Ambusher, both of which have giant downsides. And while the cards are worth more than the health choice, the fact that you can make that choice adds value.
FORCE OF NATURE
THE NERF: Summoned Treants no longer have charge, but do not die at end of turn. Card is reduced in cost from six mana to five.
Changing force of nature to five mana to summon three 2/2 Treants (that don't die at the end of the turn) changes the design of card, making it a card that fights for board and provides tokens, rather than a card that serves as the win condition for Druid.
Summoning three 2/2s ain't half-bad, but it's not something you don't just automatically throw into most Druid decks. Every Druid matchup turned into a minigame to stay above 14 health. Staying out of base combo range wasn't healthy for the game. Savage Roar can still win the game if the opponent can't deal with the 2/2s or any additional tokens, but now it's not an experience of "see Force of Nature, hit concede."
KNIFE JUGGLER
THE NERF: Attack is decreased from three to two.
Knife Juggler is a powerful early-game tool in a large number of decks - a random knife, especially playing a deck in which you're playing a lot of minions, can be really obnoxious for your opponents, ranging from mildly annoying to game-losing depending on how lucky you are. But with such powerful card text, allowing Knife Juggler the full 3/2 vanilla two-mana minion stat was just too much. Want to send a flurry of random knives at your opponent? Fine. But allowing it also to trade evenly with the vast majority of playable two-mana cards and the large collection of 3/3 and 4/3 cards at three-mana was just too good.
KEEPER OF THE GROVE
THE NERF: Keeper of the Grove's attack is reduced from four to two.
One of the long-term strategies of the design team is to reduce the fear of silence as a reason to avoid playing cool cards.
Silence has been a rare mechanic in cards released after the initial set. Introduced cards with Silence had conditions like not being able to select a single target (Wailing Soul) or only Silencing a tribal (Light's Champion).
Keeper of the Grove was another must-include because of the two very powerful options it had: Silence or instant damage. Silence has always been a problematic mechanic in Hearthstone simply because it makes so many powerful cards nearly worthless. Bolvar Fordragon had been filing complaints with Blizzard's Human Resources department for a year.
IRONBEAK OWL
THE NERF: Cost of the card is increased from two mana to three mana.
With a super-cheap, all-purpose Silence, two mana was just too inexpensive for a card that could still eat a ping or take out a 3/2 minion.
It's still playable at three mana, but now you have to choose between the Owl or the more expensive Spellbreaker, which gives you 2/1 more stats. While that makes Spellbreaker look a lot better than Ironbeak Owl, the Owl is also a nice activator for Kill Command for Face Hunters.
BIG GAME HUNTER
THE NERF: Big Game Hunter becomes a five-mana card instead of a three-mana one.
If Silence suppressed powerful cards with cool effects, Big Game Hunter provided an incredibly inexpensive manner of suppressing powerful cards, period.
While you want checks and balances to keep large, powerful cards from running completely wild, the risk of Big Game Hunter was just too large. A seven-attack card became unplayable unless the card was so overpowered that you could swallow that risk. While you want BGH around so that everybody's not just running decks full of Giants, you want the BGH player to devote more resources into such a large swing turn.
HUNTER'S MARK
THE NERF: Spell cost increased from free to one mana.
A fairly straightforward nerf here, adding a mana to the base cost. It's still very efficient-removal, allowing pretty much anything to be fought against with any on-board creature, but just a little more expensive.
BLADE FLURRY
THE NERF: Spell cost increased from two to four and the effect no longer targets enemy heroes, only enemy minions.
Well, if Tinker's Sharpsword Oil wasn't being removed in Standard play, this card would very likely have done the trick. My biggest worry here is that Rogue has become a weaker Constructed class in the current meta and Blade Flurry was one of the tools that was making Rogue playable on ladder. This is one nerf that at least on first blush, I think is a misstep - there's concern about adding large weapons to Rogue from a design space because of this card, but it means that the development team needs to take special care to watch Rogue meta developments. If a tempo, minion-focused Rogue or a return to a Backspace-esque Aggro Rogue doesn't come to fruition, there's no obvious other deck at least right now. Blade Flurry loss hurts the big combo turns of Malygos Rogue and the semi-revived Miracle Rogue archetypes. And even third-tier Rogue decks aren't likely options, with crucial cards exiting Mill Rogue (Deathlord, Antique Healbot) and Mech Rogue (The vast majority of the Mechs).
Now, there are very creative people in the world of Hearthstone, but I'm a bit worried about constructed Rogue at this moment.
LEPER GNOME
THE NERF: Attack is decreased from two to one.
Leper Gnome was a must-include in aggressive decks and as I argued earlier, must-include cards make deckbuilding less dynamic and interesting. There will always be low-curve decks to keep the greedy decks honest, but that doesn't mean that aggressive decks should practically build themselves with the choices. Everyone going against Face Hunter knows the feeling of the Hunter dropping down a turn one Leper Gnome and knowing that you already have a minimum of four damage to your face "baked in" unless you commit a lot of resources.
ARCANE GOLEM
THE NERF: Charge mechanic is removed, but health increased from two to four.
One of the lessons learned over the early life of Hearthstone is just how powerful Charge is as a mechanic. Most of the broken situations that have resulted in significant problems in the meta that Blizzard has had to address involved the Charge mechanic, from early Unleash the Hounds to Leeroy Jenkins to Warsong Commander to the overbearing Savage Roar+Force of Nature combo. One fewer Charge minion in the permanent set for Blizzard to worry about breaking the game each expansion is a good one in my eyes. And it helps with Wild format too. Without the defensive decision-making as seen in Magic: The Gathering that keeps their "Haste" cards from being too overpowered, Charge itself has long needed a structural nerf.
MOLTEN GIANTS
THE NERF: Base cost of the card increased to 25 mana from 20.
Sorry, not more free Molten Giants for you! One of the side effects of Reno Jackson having his own evolved version of Handlock is that getting those free Moltens at 10 health was less risy knowing that you were going to be healing 20 the same turn. There's still a time at which you can get the Moltens for free, but you'll have to take larger risks. And remember, the natural enemy of the Molten Giant, Big Game Hunter, is also being nerfed, so without any kind of power reduction, Moltens become scarier. I'm actually a little surprised that we didn't get a Mountain Giant nerf as well (I think Sea Giant is fine because of the conditions required).
MASTER OF DISGUISE
THE NERF: Stealth is now only given until the start of your next turn.
This is a fairly unusual nerf in that Master of Disguise never really saw any play in Constructed and only popped up in Arena when your other options were terrible. Master of Disguise never actually affected the meta, but that was because the team was aware of what a problem the card could be. Animated Armor was originally going to be a neutral card, but allowing Rogue to make Animated Armor incredibly difficult for most classes to remove was too dangerous. That the essentially game-ending interactions that were possible were hard to pull off (for example, Kel'Thuzad surviving a turn and casting Master of Disguise on him, something that could break most Heroic Adventures if you wanted) didn't mean that they weren't there and weren't dangerous. Master of Disguise had a smothering effect on the kinds of neutral/Rogue cards that Blizzard could create, which isn't healthy for the game.