Recently I got into the Hearthstone beta. It seems we're currently in the middle of a surge in digital card games (Solforge just came out, and Card Hunter, while not purely a card game, also was released recently; okay so maybe not a surge but I feel like there are a bunch out there now). Companies seem to have noticed how neatly free-to-play slots into digital card games. So far I've really enjoyed Hearthstone, and I thought I'd talk about some of the design choices I found particularly interesting or well done. My main point of comparison gameplay-wise is Magic the Gathering, which I played very not-seriously for a summer, mostly sticking to pitting different starter decks against each other (what I'm trying to say is that my experience in the genre is limited).
Hearthstone is a pretty simple game on the surface. You and your enemy have 30 health. There are cards that you play. Each card costs a certain amount of mana. Both players start with one mana. Each round players play cards that are either creatures or spells. The amount of mana each player has increases by one every round until they hit the limit of ten mana. Whoever lowers their opponents health to zero first is the winner. Of course there is a lot more to it than that, but that's the rough outline.
The games in Hearthstone are really short. I would say the average match is around 15 minutes. This is great for several reasons. Losses aren't as discouraging because they happen so quickly. You don't have to play a 40 minute game where you're sure you're going to lose for about half of the match as is often the case in a game like DOTA. Building decks is a large component of Hearthstone, and testing out decks becomes easier and less intimidating with short matches. You can throw together a deck, take it for a spin, quickly realize its flaws, and begin work polishing it.
In relation to other Blizzard games, I think one of the more subtle improvements is the addition of a "quest" system. The quests are nothing more than simple tasks like winning three matches using a certain type of deck. In single player one-on-one games it is easy to feel immense pressure to perform. If you lose it's all your fault; you have no teammates to fall back on or guide you. In Starcraft 2 this produced what was known as "queue anxiety" where hitting the button to queue up for a match was the hardest part of playing. I suffered from this, and it lead to me just watching pro Starcraft 2 matches rather than actually playing the game. The quest system in Hearthstone fights queue anxiety by giving you small, achievable goals that remove the daunting feeling that comes with queueing up. You don't feel overwhelmed trying to mentally run through your game plan while thinking about all the ways you were going to lose horribly once you did start the match, as often happened before queueing. Instead the quests present manageable goals like "I have to win two games as a Priest or Warlock today." Upon completing a quest the player gets a small reward, and by that point the anxiety has been conquered for the day and queueing up again becomes easy.
The most noticeable gameplay difference for me coming from Magic was that there was one unified source of currency for playing cards (called "mana" in both Magic and Hearthstone). The biggest benefit is that there is no chance that you'll have a bad shuffle of your deck and not have an appropriate balance of creatures/spells and the necessary mana to play them. With an appropriate balance in card costs in your deck you'll have a playable hand pretty much every game in Hearthstone.
So far I've been playing Hearthstone almost every night along with my Spelunky daily challenge. The main question remaining for me is how favored will players who buy lots of cards with real money be. Will I be able to enjoy the game without spending a ton of money on cards? So far the integration of card purchases seems very reasonable and honest. One friend hypothesized that even if you start coming up against fancy custom decks your matchmaking ranking will just fall and you'll face people with your level of cards again. If that does turn out to be the case I can see myself enjoying Hearthstone for a good while.