It's called Doom of the Last-King and it includes an entire new region set in the frozen north. The area introduces a new damage type, frost. Toons will slowly receive this DoT until they reach a warm zone, like a camp or a fire. I assume they'll be some crafted gear that helps slow down or eliminate the damage all together.
There's also fishing (meh) and a new healing class for monster play, the Defiler. All in all, looks to be a juicy installment.