When you've done the grunt work necessary to start playing the game properly, Hearthstone comes into its own. It's at this point that a little homework is necessary, and it's a far more interesting part of the game's difficulty curve than the initial level grind. You'll learn how to balance a mana curve, draft combos, how to optimise every move you make to ensure that you're leading the game, not your opponent. After a while, losing starts to feel less arbitrary and the game becomes more of a contest. Then you'll progress beyond the point where your basic cards can win matches for you consistently, and you'll go on the hunt for the expert and legendary cards that you need to take your next big step forward.
Getting more cards means acquiring booster packs, which are available for real money, in-game gold, or through the Arena. Each contains a random selection of cards, and those same cards can be broken down into 'dust' which can be spent to create specific cards. If you're after the one legendary that'd be perfect for your competitive deck, you're either going to grind for it, get lucky, or spend real money to increase your chances.
There are better business models. Hearthstone isn't pay-to-win - every card is available for free, technically - but paying does help, and more to the point when you spend you're always buying a chance at getting the card you want rather than the card itself. It feels like gambling, and that can sour the experience when you find yourself getting smacked down by someone else's ultra-rare card: one way or another, they got lucky.
The Arena is a premium mode that costs either real money or a sum of in-game gold to play. You can earn that gold by winning games and completing daily quests - 'win two games as a warrior', and so on. Dedicated players will earn an Arena run once per day, more or less. In the Arena you pick a class from a random selection of three and then assemble a thirty-card deck, one card at a time, in the same manner. It's detached from your regular card collection, which levels the playing field. It doesn't matter how much time or money you poured into getting those legendaries if they don't happen to show up in your random draft.
Once you've built your deck, you attempt to build a winning streak. Lose three times and you're out, but every win improves the quality of the rewards you'll get when you're eliminated. Gold, dust, and booster packs are guaranteed. It's more fun than grinding out boosters through regular play, but it's still a gamble.
The most satisfying experience in Hearthstone is learning to control the chaos bubbling away under the hood. When you take somebody down with a commanding deck that you put together yourself - and pulled off perfectly - it feels great. The game provides that experience frequently enough to earn a recommendation, but it's let down by unnecessary degrees of randomisation elsewhere. If there was an option to drop a chunk of change to take away the card collection aspect entirely, I probably would; likewise, if I could trade with other players to build my decks on my own terms I'd do that too.
Blizzard have the power to make the game surrounding each individual match more rewarding: to level out the game's emotional arc and give players faster access to the tremendous competition that arises when two people, all else being equal, attempt to outdo one another. As it is, Hearthstone's competitive potential is hindered by the sense that you're never quite on a level playing field with your opponent - that a little money or time or luck could always tip the odds in your favour. Hearthstone goes from good to great, I think, by embracing the idea that luck isn't everything.
A fast, deep and exciting card game let down by its business model. Great when you're on a roll, rough when you're not.