Crafting cost: N/A
Transform a minion into a 1/1 Sheep.
PW: An absolute godsend for beginners. It's not quite as good as the Shaman's Hex card, which is one mana cheaper, but it's still brilliant for dealing with beefy minions. Just don't forget that those sheep can still do 1 damage.
There's nothing more embarrassing than meeting a woolly end because you left your foe's flock unattended. Whenever playing against Mages, it's best to assume they're running two Polymorphs, because they almost always will be. Save your big minions until you've baited the Mage into using his removals on some more midrange threats.
Crafting cost: 1600
At the end of your turn, draw a Dream Card.
TC: If you get this card drawn against you late game, the jig is often up. Unless you're holding a powerful removal card, Ysera's high health pool means it will keep generating random dream cards – all of which are more effective than their mana cost imply. If the owner can keep Ysera alive, they're almost guaranteed card control in the closing stages of a match. Dreamy indeed.
P: Ysera is a bit worse than Ragnaros, but an outstanding card nonetheless. If Ysera stays on the board for two or three turns then your opponent has lost the game. Having 12 health it's usually quite hard to deal with it, especially because it has low attack. 4 attack is the most fucked up number in Hearthstone, particularly for a Priest, because it means neither Shadow Word: Pain or Shadow Word: Death can touch it, meaning the only counter Priests have to Ysera is using Mind Control or throwing away a lot of minions.
Dark Iron Dwarf
Crafting cost: 40
Battlecry: Give a minion +2 Attack this turn.
PW: Early on in your Hearthstone career you might just grow to love the Dark Iron Dwarf. His 4/4 stats make him a solid addition to the board but its his battlecry, the ability to bestow a +2 attack to a minion for that turn which can be crucial for finishing off an enemy minion unexpectedly.
Although it feels counter-intuitive you can also use that battlecry on enemy minions. Say you've got a card like Big Game Hunter which takes out minions with and attack of 7 or more, with Dark Iron Dwarf you can buff up an irritating minion like the 6/6 Stormwind Champion and then POW!
Crafting Cost: 100
Spell damage +1. Battlecry: Draw a card.
PW: This dragon has the same basic stats as the Dark Iron Dwarf, but costs one mana more. What do you get for your investment? Well, aside from being a solid mid-game minion with the magic 4 attack stat guaranteed to irk a Priest, you also get a spell damage increase and an all-important card draw.
This really is a great card regardless of deck, but the Azure Drake is especially strong in spell-centric decks and should be considered a must-pick in Arena mode.
Ancient of Lore
Crafting Cost: 400
Choose One - Draw 2 Cards or Restore 5 Health.
VS: As any card game aficionado can tell you, having 'card advantage', (i.e. more cards in your hand than your opponent), is hugely important. Ancient of Lore provides both cards and a respectable 5/5 minion.
This alone, if it were a Neutral card, would see it run in the vast majority of decks, but Ancient of Lore adds the flexibility of a sizeable heal, enabling Druids to outlast opponents in marathon matches, or buy that bit of time needed to fully stabilize against an aggressive opponent. A pair of this card should be in any respectable Druid deck.
Crafting cost: 100
Whenever you cast a spell draw a card
GW: The Miracle Rogue deck wouldn't exist without this card. Any deck that runs spells is better for it. All spell cards become cantrips [spells which trigger a card draw] when the Auctioneer is out.
The five-mana cost allows a player to easily draw at least one card on the same turn their Auctioneer is played, while the 4/4 stats makes this minion hard to remove for most classes – and, as with Ysera, a nightmare for Priests. The Auctioneer is even more effective if Rogues are able to use Conceal or Master of Disguise .
Unleash The Hounds
Crafting cost: 40
For each enemy minion, summon a 1/1 Hound with Charge.
TC: Timed right, this dog rush can be a game changer in Hunter decks because it synergises so well with so many other cards. Save it for when your opponent has board control, then combine with minions like Timber Wolf or Raid Leader to make the mutts more powerful. Also works superbly to draw cards in conjunction with Starving Buzzard , or can quickly buff a Scavenging Hyena as the hounds launch their kamikaze attack.
TOT: Combining this with Starving Buzzard, Hunter's Mark or Timber Wolf is brutal – for two mana there's no other card in the game that can clear the entire board, or deal as much damage to the opponent. It's extremely versatile. The combo works because Buzzard lets you draw a card whenever you summon a beast, and each of the hounds counts. Opponents can't really play around Buzzard/Unleash either, because if they just sit around against Hunter right, they get killed quick. So you have to play your cards, which only makes for a stronger Buzzard/Unleash combo.
Mana: 4 [Note: Altered to 5 on 22 September 2014]
Crafting cost: 1600
Charge. Battlecry: Summon two 1/1 Whelps for your opponent.
TC: Best used as a late-game closer, Leeroy is a cheap guided missile thanks to his low cost and high attack. Even more effective if you can buff his attack before using Charge, but bear in mind that if you don't kill the hero you'll want a cheap AoE card handy to clear out the two Whelps.
Truthfully, it's worth playing having just to hear him say: “All right, time's up, let's do this..." Works well in a combo with Unleash The Hounds, too.