In the play "The Crucible", what are the consequences of Abigail and John Proctor' affair?

One disasterous consequence is that because Elizabeth kicked Abby out of the house for the affair, Abby grows a violent hatred for her that eventually leads to her accusing Elizabeth of being a witch.  We see Abby's hatred in act one.  She calls Elizabeth a "bitter woman, a lying, cold, sniveling woman...a gossiping liar!"  She is mad at Elizabeth for kicking her out of John's bed and home, and also for simply being John's wife when Abby herself wants to be with John so badly.  Later when John comes in she says to him, "I'm waitin' for you every night," expressing her desire to be with him, and then she says, "I marvel how such a strong man my let such a sickly wife" determine what he will do.  Abby's hatred and jealousy lead to a direct accusation of witchcraft. She uses the needle in the poppet to her advantage, and Elizabeth is arrested.  In fact, the entire reason the accusations of witchcraft started in the first place was to cover up the fact that Abby had gone out in the woods to drink "a charm to kill Goody Proctor."  So, Miller arranges it so that almost the entire witch hunt is because of Abby's hatred and jealousy of Elizabeth, because of the affair.

Another consequence of the affair is a lot of tension and bitterness between John and Elizabeth.  In act two, seven months after the affair, they are still fighting about the situation.  The entire opening scene is them being awkward and stilted, then breaking out into a fight, over the affair, essentially.  John expresses bitterness at her cold heart that doesn't forgive, saying, "Your justice would freeze beer," and Elizabeth queries about him going into town,

"if it were not Abigail that you must go to hurt, would you falter now?  I think not."

One last consequence is John's hatred of himself, and his belief that he is not a good man.  As he is trying to decide whether to confess at the end, he admits,

"I cannot mount the gibbet like a saint.  It is a fraud.  I am not that man.  My honest is broke, Elizabeth; I am no good man."

Because of his low opinion of himself from his sins, he decides to confess.  He does change his mind, but at the beginning he feels like he isn't good enough to pretend integrity at the hangman's noose.

Those are some consequences of the affair; they are pretty severe and dramatic, and drive much of the play's conflict.  I hope that helps a bit!