The point is it can be easily countered in many situations, doesn't give you an "OMG because I have this card I'm going to win no matter what!", the card alone doesn't give you a huge edge against everyone, and probably a few more things I can't think of. Sure, in many situtations a certain card will give you a sever advantage other your opponent, but in another time, that card might not make your deck work as well. Such as against Miracle, there is no point to bring an enchant artifact and quintessence because Miracle has no perm/creature control. So it just takes up space in your deck.
I completely agree with you. I just felt the earlier response was a little too dismissive.
I can't think of a specific example right now, but hypothetically, just say there was a card that could only be effectively countered by a combination of two or three cards in the opponent's deck. Is that card then overpowered? Suppose enough people played it that it was reasonable to assume 40-60% of the decks you'd face would have that card. Technically, it can be countered, but you have to add a lot of those potentially useless cards to counter it should it pop up. So in 60-40% of games (to match the earlier assumed percentage), you've lowered your deck's efficiency needlessly just to counter a card that might have been played. Am I making any sense? Again, that was purely hypothetical.
Anyway, I don't mean to get off topic here. I just thought saying "every deck has at least one counter" is a bit of a sweeping statement, because that counter might be so specialized that it's not practical.
Right, I understand you completely. But then a second variable has to be taken into consideration. Is this hypothetical card extremely useful? Pulvy, of course it's useful, and can be counted. Using a poison strategy, there is only one single card in all of elements that can counter a good poison deck and that's purify, yet, no one really uses purify....almost at all. Purify could disappear tomorrow and only a handful of people would notice it's disappearance.
Also, sundials, hourglasses, stuff like that is useful and there is nary a reason to fully counter such a card. It all depends on a persons strategy. So what is overpowered to one person isn't necessarily so. It's just overpowered in relation to the specific strategy the respective person is using. One quick change in strategy and suddenly that card is no long overpowered, yet another card takes reign as new overpowered card. Some argue Quintessence is overpowered, other's rule Pulverizer is overpowered. A simple flick of the metaphorical strategy switch and those problems are solved while new problems take their place.
In a sense it's all about people finding that "perfect, any-situation" deck. I'm not saying everyone is looking for the absolute perfect deck....but aren't we all looking for the best deck currently possible? A few Purifies will ruin a Poison deck. An enchanted pulvy will usually ruin a rainbow deck. A firebolt/lance deck will usually lose to a creature rush deck, so on and so forth.