Blizzard confirmed that a spectator mode is coming to Hearthstone, allowing players to sit in on others' play sessions.
The news comes from BlizzPro, which talked with e-Sports Manager Kim Phan about upcoming plans for the game. Phan said that Blizzard is starting to take the e-Sports potential of Hearthstone a lot more seriously these days and has a list of features that it would like to put into the game, including a spectator mode. This mode won't be in the game by launch but should be put in "shortly after."
Hearthstone is currently in open beta.