The plot developments listed by the other educator are probably the most important in the play, but there are others that are significant as well.
A very important plot development arises from the street fight that breaks out between the Capulets and Montagues at the beginning of the play. This is a final straw for Prince Escalus, who becomes so irritated with the endless fighting that he decrees the death penalty for those who street brawl in the future. This leads directly to Romeo's banishment when he kills Tybalt.
The marriage between Romeo and Juliet is another important plot development. They marry soon after falling in love. They marry in secret, as their families would most certainly not approve the marriage. This, however, makes it all the more imperative that Juliet not marry Paris, as otherwise she would be engaged in bigamy.
A third fateful plot twist is when Romeo misses the messenger sent to tell him that Juliet is only feigning death. Had he known his, he would not have killed himself.