It should be stated on the outset that it is going to be difficult in finding a flaw in Williams' work. He is a powerfully compelling writer in displaying human conflict and interactions that help to bring out the sense of sadness within individuals. I think that this needs to be stated on the outset.
If there is a flaw in the work, it might lie in the lack of social redemption. It seems that if Williams really wants human interactions to improve and not be rooted in subjugation, there should be some level of hope offered at the end of the play. Stanley might be exposed as a brute, but he ends up being victorious. Blanche, the only threat to checking his cruelty, is institutionalized. I think that this is something that ends up becoming the ending that negates any hope of social or emotional transformation. Stella's pragmatism precludes much in way of redemption and in this, I think that one could point out a flaw in Williams' hope and what he has depicted. Certainly, those who emulate Stanley and his sense of cruelty could feel emboldened by the ending, which would be the direct opposite of what Williams wants. I think that in the lack of a socially redemptive or transformative conclusion, there might be some level of flaw present.