In my mind, the thematic qualities of "The Crucible" are what helps to define the work in a select group.  The exploration of the multi- faceted and complex nature of human motivations, emotional landscapes, and how these impact the interactions between one another creates a vibrant setting where there is much revealed.  The relationship between politics in terms of how governments can be manipulated to reflect the interests of the few at the costs of the many is also something that helps to add a powerful quality to the work.  The duplicity behind human actions in both positive and negative senses also help to make the work a very powerful one, worthy of thought and reflection.

I am not exactly sure what the question is, but if you are looking for some of the things that have helped make the Crucible a classic, they might be some of the following:

The use of the historical back drop of Salem and the witch trials which are famous.  The way that the play can portray the positives and negatives and some of the idiosyncracies of this fictional puritan community is considered one of the things that has made it beloved for so long and in so many places.

Another would be the classic themes of moral decisions and quandries as well as the way that people are so quick to become political and to often go against their own beliefs or convictions for personal gain or to maintain a reputation, etc.