The nature of the question is really broad and subjective. This means that you are probably going to get several different takes on this. I cannot help but feel that the most significant characteristic of Williams' thematic style is the emphasis of those who are marginalized. Williams really gives voice to the "fringe" or the elements of social orders that are pushed to the side. Certainly, this comes out from his own background in his experiences with his sister and his own notion of self as being gay in a society that really had much in way of repugnance for such individuals. Williams' work focuses much on how these individuals function in a rather unforgiving world. Characters like Laura or Tom, Stella or Blanche, Brick or Maggie help to bring out how Williams' style seemed to be predisposed to individuals that would have or do have a tough time finding a voice in the modern setting. Williams' style is one where the sad and delicate nature of what it means to be on the outside looking in is evoked. He does this in a manner that is not didactic. It is not in a manner that evokes pity. Rather, he does this in a style that brings out the character's own weakness while making it clear that the social order that maligns such narratives is also faulty, as well. In this, Williams brings out a state of being where there is pain and suffering all around.