Hearthstone is an online card game similar to Magic. You have a deck of cards and play against an opponent. Each player has some hit points and the person who can reduce their opponent to zero hit points first wins. Some cards attack your opponent, some protect you, some deploy minions who can attack or defend or both and some cards alter your minion's abilities. You can play as one of several classes, Rogue or Cleric or Warrior for example. Each class has special cards and abilities. The game has huge replay value based on the number of cards available and the various classes.
So here's the rub. The cards are available for purchase in virtual packs of five. You don't know which cards you'll get and some are more rare and powerful than others. The more packs you purchase, the greater the chances of getting some awesome and useful card. You can also earn virtual currency by playing the game enough times. In theory if you played all the time you could earn a lot of currency and then buy more packs without spending actual money. Keep in mind these "cards" and "packs" only exist on your computer.
In its favor Hearthstone is a fun game. It's fast and entertaining. That being said:
I like board games because they're fun, they're social, and I feel like you do tune your brain while playing. You lean to manage resources, cooperate, take chances, and focus on goals. In Hearthstone you're alone, losing a ton unless you throw down some serious time or money, and perfecting your skills at playing Hearthstone. "Hey honey, I'm going to ignore you and the kids and after three hours I'll be marginally better at a completely abstract and otherwise useless task."
So Hearthstone is gone and the next day I built a new door for the chicken coop. Whew.