Seeing as how Ix is getting nerfed and card discussions = new fad here, I've decided to start this thread.

The question is : how do I tell which cards are actually strong, and which are OP?

Take Altar of Ix in it's current state. We can all tell that it is crazily OP'd because of infinite sacrifices.

However, now take it's Brother, Nether Plasma. It is clearly only considered strong at the moment, as noone is nerfing it.

First of all that discussion was about Altar+Plasma, not Altar as it is.
Also not all top players agree that that combo is OP.

Truly OP combo/card is when opponent that see you casting he think "oh damn, I'm dead".
Main reason why I may call something OP's is when there's only few ways to counter (sometimes with rare used cards only) it and it gives big advantage.

If by OP you mean "really really strong", then there's no line between that and just "really strong". It's a continuum.

But I think this whole OP business is asking the wrong questions. Cards and combos shouldn't be nerfed because they're really strong, or really really strong, or whatever. Cards should be nerfed because they make the game less fun. With that in mind, ask yourself the following questions:

1. Is the card fun to play with/against?
(one of the main arguments for nerfing Altar/NP is that it's no fun to click pass 10 times in a row while your opponent is playing with his Plasmas).

2. To have a shot at a big EC, do you have to play the card/combo, or a deck specifically designed to counter it? In other words: does the card/combo invalidate more strategies than it creates?
The point here is that playing over and over again with, or against, the same deck is not fun.

Note that this depends on availability and perceptions. For example: it's possible that you can build CI decks that are stronger than Altar/NP decks. However, not many people do this. So going into an EC, I don't have to ask myself if my deck can stand up to a CI deck. I do have to ask if it stands a chance against an early NP, because I'm very likely to face such a deck (or 3). To an extent, the same is true for rush decks and building decks.

This might also explain why discussion focuses on the Altar, rather than the NP. It isn't about which card is more powerful. As Altren said, it's the combo that's powerful. It's about which card provides for more varied, fun and non-dominant strategies. NP enables many interesting strategies, like shield wall, siege, and various sacrifice cards. The idea is to nerf the Altar/NP combo, which is a must-use in *all* DL decks, without invalidating all those other strategies, which might be used in *some* DL decks.