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.
Honey's lost baby. Nick thinks it was a Fake Pregnancy and she just had a medical condition that made her seem pregnant for a time; while that is certainly a possibility, he never learns that she has secretly been taking birth control pills throughout their marriage because she fears getting pregnant at all, which leaves open the chance that Honey actually had an abortion (and a late-term abortion at that).
A rare example of it being done after getting awarded: It was awarded the Pulitzer Prize for Drama, but was revoked by the Moral Guardians on the advisory board.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, winning 5 - one of only two films that got nominated for every Oscar category it was eligible for (the other being Cimarron).
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.
Ho Yay: It's possible to read subtext between George and Nick, as well as between Martha and Honey. Playwright Edward Albee himself was gay, though he himself said the play was primarily an examination of heterosexual relationships. (Sandy Dennis, who portrayed Honey in the film version, has also been identified as either lesbian or bisexual by several Hollywood historians and biographers, though she made no public mention of her relationships with women during her lifetime.)
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.