Being both a Magic: The Gathering player (I've mentioned that a lot recently, damn) and an avid World of Warcraft player, I was interested to see what the deal was. Sadly, I never got a chance to find out; things seemed pretty damn confusing. A co-worker of mine enjoyed the game, even confessing that yes, things were a bit complicated at first.
You know what isn't complicated at all? Hearthstone.
It certainly helps that the game forces you to go through tutorials to help the player get a grasp on the mechanics. It also helps that the game is in a digital format instead of a physical one; you can't take any shortcuts to avoid those tutorials. It absolutely helps that the game is incredibly simple. Sure, there are some seemingly advanced mechanics when it comes to deck construction, but at the end of the day, the bottom line remains the same: Hearthstone is about finding synergy in cards without adding complex mechanics to them.
Ironically enough, another factor in Hearthstone's simplicity is something I didn't particularly enjoy in the game: the amount of cards. The game's Wiki page says that there are a total of 465. For comparison's sake, the Return to Ravnica set contains 274. Not block, but set. That's a lot of cards to consider. Now let's add in the other sets from the Return to Ravnica block: Gatecrash contains 249 cards and Dragon's Maze holds 156 cards (three of which are any good). That's a total of 679 cards in a single block. Granted, Return to Ravnica goes against the norm; Wizards of the Coast traditionally releases one large and two small sets in a block, as they did with the Theros block (579 cards total). Still, that's a lot of cards to consider when you're playing the Standard format (the most played constructed variant in Magic: the Gathering) and I haven't even added the 249 cards from the Magic 2014 Core Set.
Sufficed to say, there are a lot of cards to consider when it comes deck construction and playing. Hearthstone doesn't suffer from this problem, meaning players can spend less time worrying about what to put into their deck and more time honing a deck's synergy. It also helps that certain cards are only available for certain classes. Add in the fact that each class plays in a unique way, and making a deck becomes easier and easier. And what's the best way to bring in new players? Have a low barrier of entry.