What are the game-turning cards or combos for each class?

From experience (rank 11), I've noticed that there are a few cards that are essential to know (and to expect) from certain classes.

For example, you should expect a combo "Unleash the Hounds" with "Starving Buzzard" from a hunter at turn 5. You can also expect an AOE spell that deals 4 damages to every minions from the mage at turn 7. Both of these can easily cause a player to lose if they played into them carelessly, even if they were in a strong position before.

So I was wondering what other such cards or combinations are that can turn the game around completely and/or clear the entire board.