I have to say that I really like this question.
Arthur Miller is able to make The Crucible a frightening play because of one thing--human nature. Allow me to explain. The one thing which drives the hysteria in Salem begins with one girl, Abigail Williams. Her desire for John forces her to take very un-Puritanlike measures to draw him away from Elizabeth. She gets caught dancing naked in the woods, chanting, and making potions with Tituba and some other girls from the village.
Seeing that her reputation is on the line, Abagail begins making wild accusations about other women in the village in order to draw attention away from her own actions. As the play goes on, the villagers begin to get caught up in what is going on (kind of like mob mentality). The villagers begin to scrutinize everything that others are doing in order to keep the accusations away from themselves.
Therefore, the frightening aspect of this is that it can actually happen. Humans tend to look out for themselves. In doing so, some may make others look bad in order to make their own behaviors less "wrong." While another witch trial seems impossible, mass hysteria is not. Essentially, the play speaks to the horrific characteristics of human nature.