Thoughts from a noob 3-What makes a card overpowered?

Everybody with enough experience in any complicated card game can name a card that is powerful. It isn't difficult actually, you just have to find a card that influences heavily the board state, either because it has high numbers or an useful ability. But some cards are (or can be worked around to be) so powerful that 'powerful' isn't enough to describe them. Then we consider that card as overpowered. The card is just not fun anymore because it lets you win too easily. I thought I could make a short list of characteristics a card could have to cross that line and see what we can do with them.

1-Your opponent(s) have very little they can do.
A card can be named overpowered if it lets you win with a low chance of something going wrong. Luckily, that doesn't exist in GOTLCG because you can Tears of Lys anybody, use Core Robb to neglect Stannis, other Robb or Areo to uncommit core Balon, PttT or WdnS to discard The Wall...etc. These kind of options exist in all games for what I know, but a sweet spot in this regard are cooperative card games like LOTR and the recent Arkham Horror, where the cards are pulled at random so they need to be balanced. That makes starts the critical point of the game while finishes are just abot killing everybody with your 20 cards.

2-It makes other cards far more powerful too.
Another way a card can be overpowered is if it also increases the power of other related cards. Let's consider core Stannis. Robert, Mel, Barry, Followers and EHJ are good on its own, but add the Lord of Dragonstone to the equation and suddenly your opponents have all their guys knelt while you attack them over and over again. I know cards are supposed to synergize with each other, but sometimes when it comes this powerful it makes me wonder if they put it a bit over the limit.

3-It limits the future cardpool.
Do you know why Renly is the first big guy Tyrell has received since the Core Set, while every other faction is swimming in Hounds, fat Cats, Kings and Ser Barristans? I think it's because of the Queen of Thorns. Her ability is just so powerful that the designers have limited the number of good targets for her, making Tyrell grow weak(pun absolutely intended). It's just my theory, but if it's correct it is a clear example about how a single card can drag down a whole card pool. The other option is to ignore that case and still making powerful, synergizing cards. This happened to Boromir and Dáin Ironfoot from LOTR. The latter was inmediately followed by a whole cycle and 3 big boxes of dwarves, making Dáins dwarven-boosting ability really strong. The former took a bit longer, but ultimately there are so much options to counter the cost of his ability that he's just insane (and if you don't believe me:).

4-It makes people write stupidly long posts about them.
You're now reading one of them.
...
Okay, I just wanted to put 5 on the list.

5-It makes the game not fun.
This is the most important of my points, because why are we playing games, if not to have fun? We just want to have some fun while playing a game with someone, and anything that makes a game be heavily balanced towards one side limits that fun(both for the losing player, who would prefer having a chance to turn the tide to its favor, and the winning player, who could prefer a more close victory). I don't want to change anything about games, I'm just saying most people, while having a vast array of personalities(Ned, Shagga, Jaime), are in games for one main reason: to have fun.