Game balance: Specialized and unspecialized abilities

(This is a post about game design. It's generally targeted at game designers, although players may also be interested. It assumes some knowledge of common computer game genres.)

In Hearthstone, there's a card called Auchenai Soulpriest. It turns all your healing spells into damage spells. It also has a 3/5 body, and it costs 4 mana. (If you don't know Hearthstone, that's okay – for this post, all you need to know is that it's a game where you choose what cards to put in your deck, bigger numbers are better, and spending more mana is worse.)

Auchenai Soulpriest is a well-balanced card.

But there's another card, Embrace the Shadow, that does exactly the same thing, except that it costs 2 mana, only lasts one turn, and doesn't have a body at all. So when you play Auchenai Soulpriest, it's like you're playing Embrace the Shadow and also paying 2 extra mana for a 3/5 body (and a chance for the effect to last longer). Adding a 3/5 for 2 mana is normally an outrageously good deal. You'd normally have to pay 4 mana for that. So it seems like Auchenai Soulpriest was balanced as if Embrace the Shadow was a zero mana card. Why?

Specialized and unspecialized value

Here's why: The strongest way to [...]