A new Paladin hero skin was added to Hearthstone and it can be unlcoked by playing World of Warcraft. Lady Liadrin can be unlocked by leveling up a World of Warcraft character to level 20. Even if you are not subscribed to WoW, you can level a character to 20 in the free starter edition. Note that this is not retroactive, so if you already have a level 20+ character, you will have to level one more.
Along with a new hero, the recently released Hearthstone 4.2 patch added 9 new deck slots, a few new cardbacks, and other minor updates.