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I've played all kinds of CCG over the years, on- and offline. Among them Magic, Magic Online, Rage, Legend of the Five Rings, Vampire, Shadowrun. And recently I had a look at Hearthstone. The game is currently in closed beta, soon in open beta. But since you can already spend real money for it, let's have a look at what's good and bad about it.
Now, first, what I like about Hearthstone:
Graphics: They're good for a CCG. The art has a high quality, the spell effects are fun to watch and the animations (cards moving around) look fluid. In addition, the backgrounds of each match have a setting you might know from the Warcraft universe, that is also interactive (like, you can smash a window of a building seen in the background by clicking on it several times, or load a catapult with a rock and shoot it over to the enemy). If you hit an opponent with your creatures, then, depending on the damage dealt, you get different sounds (crowd cheering), animations, shaking screen and so on.
Sound: Also good. Each creature has some sound sample taken from World of Warcraft. The music runs pleasantly in the background.
Interface: it's clean and simple, easy to understand.
Complexity: also very easy to get into. Most of the stuff explains itself, a few things are found out by trial and error.
Game length: seems reasonable too, something between 10 and 20 minutes (though exceptions are of course possible).
Cost: it's free to play, you don't have to spend money (though you can). You get new cards mostly by buying booster packs for the ingame currency, that you can get for completing quests and achievements (there ain't that many achievements, quests give you some of that currency once per day).
Crafting: you can destroy cards you don't like for crafting materials to craft cards you want.
What I don't like so much:
It's pay 2 win: rare cards are in general better. Much better. The chance to get more rare cards increases with the number of opened booster packs. If you do it by questing, you open one pack every two days or so. Of course, if you spend money to buy boosters, you get many many more cards, more rare cards, and can destroy the unneeded ones to craft missing ones. So, you will lose to opponents who spend more money for the sole reason of them spending more money.
Stalling opponents: normally games have a hidden turn timer that tells you when your turn will end soon. Sometimes though (I guess when choosing a target or something) the timer doesn't show up. I once had an opponent about to lose a game, and while choosing a target he went afk. I alt-tabbed out, played some other game for an hour, came back, and he was still there. Seriously: you do not want to lose against opponents who win just by indefinately stalling the game.
No report option: you can't report players displaying toxic behavior.
No chat: Apart from using some predefined emotes, you can't communicate with your opponent. For me, such games are also about making new friends to play the game with. You can't say "afk for a minute, telephone" or "where did you get that card from" or "can you please hurry up" or anything else like that. There is basically no difference whether you play vs humans or a good (or sometimes bad) computer.
Only two players. While games like Magic Online support matches of up to six people, Hearthstone only supports two people. Which is quite a shame. While such matches would take a while, they'd also be quite entertaining. There are no team modes or FFA modes or anything else.
Few different modes of play: you have practice vs a normal AI, vs a better AI, normal matches vs humans, ranked matches, and some draft (where you build a deck from the scratch, costs money or ingame currency though). So in short, except for the draft, you always use the same decks. Which is also a shame. I liked that in other games, you have different formats to freshen up the game. Like a format where you have huge decks of a thousand cards, or decks where no card can be played more than once, decks that force players to run certain themes or have another certain type of structure. But in Hearthstone, it's always the same decks.
The hero system forces people to run similar decks: for each deck, you choose a hero. That hero gives you a special ability that you can activate once per turn (like dealing 1 damage to any target, healing a target for 2 health and so on) and access to certain additional cards. Since you want to make best use of the heroes special ability, and don't have access to all cards (only the specific hero and the general cards), the number of strategies is very limited. People play somewhat similiar decks over and over and over. If you see a card specific to one hero and one specific to another, and think that combining them would enable you to create a great creative deck: no chance, can't do that, you have to stick with the regular stuff.
No trading: you can't trade cards with other people. Which sucks of course. You got the one demon I want for my warlock deck? Hey, I got the one other dude you want. Let's trade? Nope, we can't, no trading possible. You're stuck with your cards, I'm stuck with mine. The only solution: buy more cards for real money or craft the cards (though crafting requires you to rip apart many many of your cards in order to be able to craft a new one). And the cards you tear apart to get crafting materials have to come from someplace too, which is, the shop (questing just takes a long time). Also, no trade means that cards have no value. You can get the most awesome super rare card in a booster pack, but it is still without any value but to your decks. In certain other games, if you find a card that is high in demand, you might be able to sell it for 30$ or so, on ebay or trade sites or whatever. Cards have value. And you can gain new cards by clever trading. Not so in Hearthstone.
The game is fun, well made, easy to get into. You win some early games easily, do quests, get achievements, gain ingame currency to buy boosters, rip them open, get new cards, and so on. It's awesome. But as you progress, things get somewhat boring. Variety of decks? Low. Different game modes? Not really. Trading? Nah. And to that the strong pay 2 win mechanic, and you'll be playing something else after a week or two.