Lately I've been playing a lot of Hearthstone, and occasionally watching my son play Hearthstone. As an online CCG it has some really great qualities, such as how easy it is to find a game, the different formats available for play, and the way I don't risk personal exposure to the ass-cracks that play meat-space games like Magic: The Gathering. A friend of mine, that's also interesting in game design, proposed it as a mutual reference since he's primarily interesting in video game design and I'm more interesting in board games and tabletop wargames like Warhammer 40,000.
The games have quite a lot in common, with minions in Hearthstone corresponding somewhat to units in Warhammer 40k. Obviously there's an extraordinary number of differences, of course, but as someone else pointed out the CCG, and particular Magic: The Gathering, has really set the tone for gaming. Magic, and Hearthstone, are about constructing decks as much as playing them. Indeed, some formats involve building a deck as you play. Warhammer is something like that, in that you build an army, and there's a 'list-building phase' prior to the game. And afterwards you sit and think about how it would have been different if you'd gotten certain cards you'd left out of your deck. It creates an interesting cycle of build, play, and collect. Similarly in Warhammer one is often left feeling like the game would have turn out differently if you'd bought/built/painted a particular unit instead of one of the ones you actually brought. Of course, one of the important difference between Warhammer and a card game is that units are composed of miniatures that you build and paint. They're a lot more labour intensive, and a lot more personal than simply owning a copy of a card. Getting cards you no longer play because they're below the power-curve is of little consequence, especially if you can break them down to build new cards as one can in Hearthstone. Getting miniatures all prepared to find that they're below the power curve is much more disheartening because it's rather personal to find out what you like is a handicap to winning, and as much as I like to emphasize that I enjoy a good game winning or losing, I think a close game is best and going into a game with bad odds does not make for a good experience. I'm not particularly keen on 'clubbing baby seals' either, as the vernacular was not so long ago.
Hearthstone is different from Magic in that you have a steady progression of 'mana' or the resource that allows you to play cards; cards are costed in mana and that provides a hard and fast limit on the number that can be played on a turn. Hearthstone would be a very different game were any and all cards able to be played on the first turn, as frequently is the case for 40k. In 40k, the game wrestled with the issue of an "Alpha Strike" essentially handicapping one player from the start of the game. This was somewhat resolved in the 5th edition where the players rolled off to see who went first, and then the player that won the roll decided to either deploy first and take the first turn, or deploy second and cede the first turn with an opportunity to seize it on a dice roll. The balance, supposedly, was that you could deploy first knowing that your opponent wasn't going to set up a shooting gallery for your first turn pleasure, or second and hope that you could either seize, or more importantly knock your opponent off their objectives at the end of the game.
The 'standard deployment method' in 7th edition of 40k is a roll-off to determine who sets up first, with the winner making the determination. The players that deployed first then decides where to take the first or second player turn of the first game turn. The player with the second turn gets to try and seize the first player turn. Decoupling the option of deployment from the determination of first turn means that the player that deployed first may be able to seize an opportunity, or bide their time. In Hearthstone the first turn is determined randomly, with the second turn getting an extra card and a Coin card which is a spell (thus triggering spell-conditioned effects) that gives the player extra mana. It's generally considered something of overkill compensation for the second turn, but the granularity that makes it overkill is also responsible for an interesting facet of game-play whereby a player tries to kill their opponent early in the game with weak, low-cost cards. Proportionately-speaking, low-cost minions are often proportionately more powerful, particularly if they're more than their Attack and Health. Any card that summons multiple minions has an advantage as well, like 40k, because a player's ability to engage them may be limited to fewer than the number present on the board, providing what in 40k is known as target saturation. Some minions have a 'Deathrattle' effect whereby they do something when killed, which is under-exploited in 40k (vehicle explosions, Blood for the Blood God), and some of those summon more minions, effectively doubling (or sometimes tripling) the number of targets a player must engage to retain board control.
Which brings me to some notable differences between 40k and Hearthstone, in that 40k lacks a 'face' to be attacked. In Hearthstone you can attack characters, including minions and the hero or 'face'. In 40k units can only attack other units, which is interesting in that sometimes they cannot even do that when they're caught out of position. Conversely, Hearthstone lacks the game of position that 40k has, with all characters able to engage each other, with a rule, Taunt, demanding some target priority. Of course, in Hearthstone it's simple trumps to resolve the conflict: A minion's (or weapon's) Attack is subtracted from the target's Health. At zero health the character is removed from the board, triggering Deathrattles in the case of minions, and losing the game in the case of Heros. In 40k every attack is a risk, risk at the level of hitting, wounding, and saving (and then rolls like FNP and Damage), and some attacks are one-sided in the case of shooting, or carry the risk of a return attack in the case of close combat. All attacks in Hearthstone go both ways, although a minion or hero attacking face won't be affected by any weapon that the Hero has equipped. That one-to-one ratio though, is the important thing these games have in common, and even then there's exceptions.
In Hearthstone there's spells and effects that affect more than one character, and sometimes in complex ways (like Swipe, which is four damage to a character and one damage to all other characters). They're sometimes called "area of effect," and 40k shares these in some ways too, with blasts and templates allowing collateral damage to units other than the target, and with close combat allowing a unit to engage multiple enemy units in combat (with the disadvantage of slightly fewer attacks than they would normally get from initiating combat). Friendly fire (sometimes literally) can be a feature in both games.
Getting back to the beginning of the game, both games feature the deployment of units/minions, as well as the use of spells in Hearthstone that could be said to correspond to off-board effects in 40k (and which 40k lacks in its standard formats, although it strikes me that off-board artillery not depending on any on-board units/models would be great for commensurating armies of different points-values). Hearthstone does a kind of drip-deployment, whereby mana increases by one available point or crystal per game turn, although there's spell/minion cards that allow players to variously increase or decrease available mana. In the standard progression though the game slowly allows spells and minions of increasing power and effect to be deployed. Players pour more and more spells and minions onto the board in the attempt to kill the other player's Hero first. By contrast in Warhammer 40k players typically start with their entire armies, collections of units equivalent to decks in Hearthstone, on the board and the game progresses as an attempt to clear the board most efficiently. The 6th edition of 40k introduced progressive scoring in the form of the Maelstrom of War missions, whereby players had a pack of cards from which they could draw a hand of objectives to score as the game went on, and previous editions were scored when the game was over leading to issues with the aforementioned deployment/first turn issue in 5th edition when that was introduced to deal with the issue of the alpha strike.
Hearthstone deals with the alpha strike, though not entirely successfully, by throttling mana. This opens up space for the cards, usually for the Druid hero, to increase the amount of mana available to the player and hence an early-game advantag. Now it's not entirely necessary for a 40k army to deploy in full, but if no models from an army are left on the board at the end of a game turn it's a Sudden Death Victory for the other player. That pooched so-called 'null deployment' lists from earlier editions that avoided the alpha strike by not being around on the board when it could happen. Which brings me to another notable difference between Hearthstone and 40k, that in Hearthstone terms all 40k units have a Battlecry because they can shoot when they enter play from reserves, essentially the player's hand/deck in 40k, but they cannot typically charge. Some units straight up have Charge, like in Hearthstone, and the inability to charge from reserves in general was introduced in 6th edition, and has been slowly re-introduced in the form of special allowances for certain units rather than as a general thing.
Of course, 40k is limited to 5-7 turns, whereas a typical Hearthstone game counts that as the opener. The reversal in the number of elements, units or cards in play, perhaps has something to do with it. A Hearthstone game might last ten-fifteen minutes, whereas a 40k game can last more than two hours. Lack of automation might have something to do with this, as an app that figures out and executes all the dice-rolling automatically would be a huge time-saver. People think it's the movement phase, and it can be, but the real time-waster is dice-rolling, and Hearthstone solves this RNG issue in the back end so you just see the result. Nonetheless, while I would like to see something of a drip-feed in 40k, previous attempts as so-called "escalation"-style play was unsuccessful because a strong part of 40k is the co-relative positions of all the units on the board. A 40k game involving one or two units in play is kind of boring, which tends to make very small games a very dull affair. In Hearthstone's case the ability to pull off an aggro style of play that knocks out the other player in the early game is part of what makes the game interesting in the early game as players struggle for board control, which is one reason why low-mana minions are disproportionately powerful in that they have a head-start and each turn they're on the board is literally a force multiplier.