I think that the poppet scene truly reveals the level of antagonism that Abigail has towards Elizabeth. It is also the moment when Elizabeth's suspicions are no longer paranoia. The poppet reveals the extent to which Abigail wants Elizabeth gone, in order to accomplish her own ends of snaring John Proctor for herself. The significance of the scene reveals much about both characters. For Elizabeth, the poppet scene brings to light the fundamental "crucible" that she and her husband must face. Due to his own transgressions, John has endangered Elizabeth. He comes to recognize this as she is being arrested, as it is the first moment where we start to see him take an active role in defending his wife and his own state of being in the world. The poppet scene is also significant because of Abigail, herself, and what it shows. There is a level of deviousness that is evident in Abigail's planning and machinations in order to accomplish her desired end. She planned the poppet, was able to manipulate Mary Warren in order to accomplish her end, and proved skilled and being able to take random events and link them in a sequence to make her the victim and Elizabeth the aggressor. It is through the poppet scene where Abigail's personality is reflected to be quite an intense one, and one where she is quite dangerous.
As a result of Abigail's claims about the poppet, we (and John) have the chance to see that Elizabeth was right about Abigail all along. Up until now, John sees Abigail as just a girl, but Elizabeth knows what ruthlessness she is capable of, that Abigail still harbors strong feelings for John, and what terrible things she is capable of in order to see her dreams of being with him again come to fruition. Thus, we learn that Elizabeth was never paranoid; her suspicions of Abigail were absolutely founded and valid. We also see John wrong, again. He was wrong, morally, before the play began when he bedded Abigail, and he is wrong now when he denies that Abigail would try to harm Elizabeth. We might forgive him his transgression of passion; we might empathize with him in regard to his affair (as Elizabeth later does). However, his failure to protect his wife and his town when he doesn't report what Abigail told him is more difficult to understand. It is in the poppet scene when he realizes the full extent of his mistakes. With the revelation that Elizabeth is right, he becomes more difficult to redeem in his own eyes and ours.
Further, the poppet scene helps to show us how well and truly hysteria has taken over the town and how eager the townsfolk are to believe anything the girls say, no matter how outlandish (or the evidence to the contrary). Mary Warren made the poppet and stuck the needle in; Abigail sat next to her in court when she made it; there are no other poppets in the Proctor home but the one Mary made: there is plenty of reason to believe that Elizabeth is innocent. On the other hand, there is plenty of reason to believe Rebecca Nurse is innocent as well. It doesn't matter. As a result of Cheever's behavior during this scene, we see how a mere accusation is now enough to presume guilt.