In Hearthstone’s normal play mode (Constructed – pre NAXX), every card you include in your deck cannot exceed a total of 2 copies of the same card, and you have a limited amount of cards in your hand.
So, under those conditions, in which turn can you win if you take the enemy down as quickly as possible and the enemy doesn’t concede ?
A win on your first turn is possible if you play as a Druid against another Druid who plays Millhouse on their first turn.
In the video below, the opposing druid opens with an Innervate and summons Millhouse Manastorm. Millhouse’s battlecry says: “Enemy spells cost (0) next turn.” (Which means your spells cost (0) on your opening turn.)
Next, the player uses this opportunity play just shy of their entire deck and win. A pivotal card is Gadgetzan Auctioneer, which gives them a card every time they play a spell – giving them a virtually endless supply of 0-mana spells.