According to the following internal email, it seems that Blizzard knew they were onto something big when their own devs had to be pulled away from playing Hearthstone during the game's development.
Dustin Browder, who was lead designer of Starcraft II before taking up the position of game director on Heroes of the Storm, sent the attached message to lead designer Eric Dodds in September 2012 (which was more recently shared by Blizzard's Dave Wilson on Twitter).
It seems that the first people to deal with an addiction to Hearthstone: Heroes of Warcraft were from within Blizzard. Given what we know now, it's hardly a surprise.
New content for the CCG was detailed during this month's Blizzcon. You can read our preview of the incoming expansion - Goblins vs Gnomes - by following this link.