I have been thinking a lot lately about the way Blizzard costs cards in Hearthstone. I have been looking to figure out my own formula and use it to describe why cards are powerful and get a grip on what a particular change to a particular card would do to its overall power level and its chance of getting played. The really obvious card that makes this analysis interesting is Piloted Shredder, the ubiquitous 4 drop that is used in every kind of deck.
People love to hate the Shredder because for good or for ill it has nearly ended the use of all other 4 cost neutral minions. The question is, how much better is it exactly than comparable other options, and what could be done to reduce the Shredder to similar strength to all of its compatriots like Chillwind Yeti, Sen'Jin Shieldmasta, and others?
My idea is this: Cards should be evaluated based on what usually gets played. For example, a 3 drop 3/4 is bad and isn't played - there needs to be something else. Dark Cultist has a useful deathrattle, Spellslinger has a useful battlecry, Totem Golem comes out a turn earlier, Animal Companion has more stats, and Spider Tank can leverge its Mech type. All of these cards I will assign value 3, as they are strong 3 drops that are regularly used. A vanilla 3/4 isn't good enough to be played, so it has a value of 2.5. It would be a crazy 2 drop, or a terrible 3 drop. A 2 drop looks like King's Elekk, Knife Juggler, or Armoursmith. 5 stats and a good special I assign value 2.
So, given these benchmarks lets look at some cards. We have vanilla minions that draw 1 card (1/1 for 2, or 2/4 for 4) and they don't see much play. A 1/1 costs -0.5, and adding a card draw is worth 1.5 mana for a total value of 1 mana. A 2/4 costs 2 and with that same value for the card is worth 3.5 mana. In both cases they are weak for their cost, and as expected they see little play. On the other hand we have Azure Drake, who is actually good. 4/4 is worth 3, spellpower is .5, and the card draw is 1.5 so Azure Drake is worth 5, and in fact it does see quite a lot of playtime as a result.
Now let's look at the family of minions that gives you an extra minion when they die. Haunted Creeper is first up. Those 2 1/1s are worth .5 mana, and a 1/2 is worth 0 mana. However, you don't have to spend 2 cards to get those things, so we credit the Creeper with 1.5 mana worth of cards drawn. This leaves us with a total value of 2, which as you would expect means that the Creeper is a solid 2 drop.
Harvest Golem gives a value of 1.5 for the basic 2/3 body and 0 for the 2/1 that follows. Add in the 1.5 for the extra card we are getting and the Golem clocks in at 3, which is exactly its cost. This suggests that the Golem is a strong 3 drop, and although it doesn't see a huge amount of play it is definitely worth including. I think that is accurate, though perhaps a slight overstatement.
Now for Piloted Shredder! A 4/3 is a bad 3 drop, so we value it at 2.5. A random 2 drop is a lot worse than the sort of 2 drop people normally play, so it is only worth 1.5 mana. However, because we are getting two cards out of it we again add 1.5 for the card and arrive at a beastly 5.5 total. I personally feel like that is too charitable and that we should subtract another .5 from the total because the random 2 drop can actually be a huge detriment or just plain awful, and 4/3 is a really bad stat spread. Even then, Piloted Shredder is a 4 drop minion with 5 mana worth of value baked in, which definitely makes all the fuss seem justified.
So, given that Piloted Shredder is clearly over the top compared to other 4 drops both by popular play numbers and by my 'scientific' analysis, what could we do if we wanted to make Shredder more in line with other cards? My thought is that it should become a 4/2. That preserves the Shredder as the go to choice for aggro decks as they want to bash hard or mech decks because of the mech type, but it suddenly becomes a lot more vulnerable to 1 and 2 drops and AOE effects killing the main body. At that point it looks pretty mediocre for a control deck, and even midrange decks would have to consider if they actually want it over a Chillwind Yeti. I suspect they still go for Shredder because of the strength against direct removal and board sweepers but it is a close call at that point, which is exactly what I was aiming for.
I should note that I don't think that the Shredder is a huge problem. It is in a lot of decks but nowhere near all of them. However, it is definitely the top of the heap and any new neutral 4 drop is going to have to be absolutely insane to compete. You gotta know that a Mech statted for beatdown that appears in non Mech control decks has something sketchy going on there! If I were in charge I would probably tinker with it because of that limitation. However, I don't think that nerfing Shredder is necessary as Blizzard does want to have a light touch that way.
Dr. Boom though, that guy needs the nerfbat in the worst way. (Boom bots should do 1-2 damage instead. Still great, but less ludicrous in the extreme cases.)
Note that my formula just doesn't work when you get up to really high cost minions. It seems to generate good results in the lower costs, but I think Blizzard really just eyeballs it when things cost 6 or more.