Well if you think about the basic idea of the game, it's choosing the best way to get the most Victory points the fastest. You win by choosing your cards carefully. It is in the choosing that determines who is the better player. For this reason, when considering balancing options, I look at cards that are 'must buys' because these simply do not belong in the game. If there are cards that need to be bought or you lose, then the game's philosophy is flawed. The balancing in this game, overall, is exceptional but I found a couple of cards that are bought a little too often
.
Chapel - This card is severely under priced at $2. There are more threads on the rare occasions that you should
not pick up a chapel rather than ones where you should. I buy it in nearly every set up of cards that includes it. Raised to 3$ would solve the 5/2 Chapel imbalance. Raised to $4 might have people contimplating not picking it up. Reducing 4 trashed cards to 3 could work too.
Mountebank - So powerful that if someone gets to $5 first and doesn't buy it, I'm fairly confident I have won the game. Raised to 6 would have people contemplating it with Gold. Perhaps reducing +2$ to +1$ could help a lot too.
I can't think of too many other must buys.
I guess you could go to the other side of the spectrum and look at cards that are never bought, or never buys. However at the current state of the game, it is rare that there are cards that are never actually helpful. The only card that I think that could fall into this category is Pearl diver. It's relatively weak but at $2 it can give you some useful deck information.
Woodcutter is weak overall, but in a game with no other +Buys it becomes a good buying option.
Chancellor has a niche with Counting House, other than that, it's not an amazing card.
These two cards are weak but there are a couple of situations where they are decent, if not good buys. Which is why I don't think they're imbalanced, just a little weaker than other options.
I guess the point I'm trying to make here is that there shouldn't be cards that you feel you must buy(or must not buy) regardless of the set up. If there are, those cards are imbalanced.