Opportunity Cost
Narrowness
Just plain badness
I would divide this differently.
I don't think any published cards are so bad that I never want them. Some come close. Anyway "just plain badness" to me is just not coming out and saying the real reason the card is bad.
Meanwhile I would divide up opportunity cost into two things. And then I have a different third thing, which is related to narrowness but a better category. Then we can note that cards are only weak relative to other cards (though those other cards include base cards).
A card is weak when it's costly, relative to other cards, in one or more of three ways:
- the cost to get the card
- the cost to play the card
- the cost to support the card
The cost to play Pearl Diver is low. If you Ambassador me a Pearl Diver you are helping me a little bit. The opportunity cost to buying it is too high though. We compare Pearl Diver to other cards you can afford, and immediately it's a weak choice unless you just have $2, and even there it's not great. Note that cost-to-get includes a buy; it's not that Pearl Diver becomes good at $0.
Chancellor meanwhile has a low opportunity cost to get; it costs $3 and makes +$2, like Silver, and you often pick up at least one Silver with a $3 hand, and can just get Chancellor instead then. Its cost is for using it: it eats an action, and you can only have so many terminal actions, or need to spend time supporting more. For what Chancellor does, the cost to play it is usually too high relative to other cards. We compare Chancellor, not to other $3's, but to other terminal actions; most terminal actions are better, even (and especially) ones that cost more.
Those are the two kinds of opportunity costs: the cost to get a card and the cost to play it. You get to play one action each turn and buy one card! It's those two costs.
There is a third factor though, which is that cards vary in power level with the rest of your deck. Coppersmith requires Coppers; to have it be good, you have to draw enough Coppers with it, which means not trashing those Coppers, having card-drawing, supporting the card-drawing (or, getting rid of the Coppersmith when your deck stops being good for it, since your deck initially draws lots of Coppers). It's not just about spending an action on playing it and a buy phase on buying it; it costs more than that. Pearl Diver and Chancellor have much lower costs here; I might draw them dead with a Smithy but that's about the extent to which my deck needs to be something special to have them.
These three costs vary from game to game. Your opponents affect them too. The cost to play a card is never completely out of reach (though something else may be much better), but the cost to get a card can be, and the cost to support a card may be obviously out of reach from turn one.
It's easy and convenient to think about cards as if you are buying cards, to do your own thing. In fact you are building a deck, and playing it against opponents. Scout isn't weak because of the cost to get it or play it; it's weak because you need the deck for it. Thief is weak because your opponents don't tend to make it good for you. Some cards do end up weak just because of opportunity costs, like Pearl Diver being too costly to get and Chancellor too costly to play; but obv. other factors can still boost them, e.g. Mystic, Stash.