Various theories, a popular one is that Martha is the only true person in the play, she's imagining all the others.
There's evidence that George is gay, having married Martha to try to advance himself. There are subtle moments when it seems George is hitting on Nick. Then again, just about everyone has a moment together at some point.
A rare example of it being done after getting awarded: It was awarded the Pulitzer Prize for Drama, but was revoked by the Moral Guardians on the advisory board. The film, on the other hand, was showered with 13 Oscar nominations.
Some consider the movie a minor example for losing most of the big awards to A Man for All Seasons. A fine film in its own right, but about as safe an Oscar choice (British-set period piece based on an acclaimed play) as one can imagine, especially next to Woolf.
Jerkass Woobie: All the characters have no problem insulting and attempting to humiliate each other, but deep down, we see that they are just miserable, sad people, trapped in their unhappy lives.
Nightmare Fuel: This first came out during the Cuban Missile Crisis. People expecting to escape from the most terrifying two weeks in history instead found themselves confronted by (then) shocking language and one of the most depressing depictions of marriage ever.
Tearjerker: Martha's freak out towards the end when her and George's "son" has been killed off, the pain emphasized further with Elizabeth Taylor's excellent acting in the 1966 film adaptation.