The thing with the pressure points is that they are entirely based off Magnussen's impression of a person. They're a personal list, for his use, of what he can use against a person. John's list of pressure points is short: Harry and Mary. Why his sister (who he doesn't get on with) and not his best friend? Possibly because there is nothing he can do to Sherlock anymore that will force John's hand. John is used to Sherlock getting the two of them into dangerous situations. If Sherlock were to be kidnapped by Magnussen, I don't think John would react as strongly as he would if it were Harry or Mary, because he sees Sherlock as this indestructible force.
Sherlock isn't John's weak point, and it's not because it wouldn't upset John if he got hurt. It would upset John very much indeed. It's because John is calm in the face of danger, especially Sherlock's danger, and he does not take prisoners. He trusts implicitly that Sherlock will do what needs to be done to fix things. And there is nothing Magnussen can say or do that will shake this faith, this almost irrational faith, that John Watson has in Sherlock Holmes. There's nothing to be used, so there's not point in listing it.
But John - John is the only person that has ever been willing to have Sherlock as a best friend. There is a reason John is on the list of Sherlock's pressure points. Sherlock will kill himself for John. He will destroy himself for John. In the game that Magnussen is playing, that is how the pieces fit. And that is how he intends to play them.
Re: So, His Last Vow and the entire S3 feels
Sherlock isn't John's weak point, and it's not because it wouldn't upset John if he got hurt. It would upset John very much indeed. It's because John is calm in the face of danger, especially Sherlock's danger, and he does not take prisoners. He trusts implicitly that Sherlock will do what needs to be done to fix things. And there is nothing Magnussen can say or do that will shake this faith, this almost irrational faith, that John Watson has in Sherlock Holmes. There's nothing to be used, so there's not point in listing it.
But John - John is the only person that has ever been willing to have Sherlock as a best friend. There is a reason John is on the list of Sherlock's pressure points. Sherlock will kill himself for John. He will destroy himself for John. In the game that Magnussen is playing, that is how the pieces fit. And that is how he intends to play them.