So we all know that balance is a huge issue when designing dominion cards. You can't make a card that is so powerful that you play it all the time, and it totally dominates everything (even though it might be fair, because everyone has equal access), because that makes things very formulaic, and not very fun. Similarly, you can't make a card that's so weak that you never ever ever use it, because that's really boring too. Okay, scout got printed. But you know what I mean.
But there is this other issue that is absolutely related to this, which might even be worse. And that is the card which, most of the time, is basically worthless, but in that pretty rare occasion where it's useful, it's game-warpingly dominant. This is what I call FBI - fancy balance issues. I know from first-hand experience, this is really easy to do. And the tricky thing is that, on average, the card is at a reasonable power level. It's just that in any individual game, in every individual game, it is not.
This leads to another tidbit of note about power levels; there are ten kingdom cards in every game, and most often, most of them are not going to be used in the optimum strategy, and that's okay. If a card is useful in half the kingdoms it is in, that's great design. If it's only good a quarter of the time, that's fine too. So long as it's not degenerate in those cases.