O'Neill's Long Day's Journey into Night is a modern tragedy in form and function. The original, classic tragedy was defined by Aristotle and exemplified in ancient Greek drama. This play would feature a hero of noble stature who experienced a fall from grace due to their own flaw. The fall was supposed to be so dramatic that it would bring about a sense of catharsis or emotional release from the audience. The audience was also supposed to learn from the experience, believing that they could easily trade places with the hero and fall in a similar manner if they were not careful.
Sophocles follows this format with Oedipus Rex. Oedipus has gained stature by solving the riddle of the Sphinx and saving his people, but because of his pride and arrogance, he fulfills the prophecy he sought to defeat, killing his father and marrying his mother. He becomes an outcast and ends the play blind and alone, having lost all he once had.
On the surface, O'Neill's play does not seem to exhibit the characteristics Aristotle defined, yet careful study reveals it does have them, only in a more modern sense. For instance, we have a family with two central figures, James Tyrone and his wife Mary, who begin their lives together with dignity and fame. When we meet them, they have already fallen, but they do experience a fall from greatness. He was an actor with potential for greatness, and she was a sophisticated young woman filled with promise. After they experience the tragic loss of one of their children, they spiral into a cycle of self-blame, self-loathing, and general finger-pointing that leaves their other two children unable to grow and develop. Mary blames James for being too cheap to pay for a good doctor, and James blames Mary for her inability to fight her addiction to morphine. While we don't see the dramatic finish we see in Oedipus Rex as the titular character tears his eyes out, the audience does experience catharsis. The final scene with Mary in her wedding gown lets the audience know this family will never be able to move on from its painful past.
So, while O'Neill doesn't offer a traditional tragedy along the lines of the Greek form, he does offer a modern tragedy, one that leaves readers believing indeed that they could easily suffer similar consequences should they fail to face their pasts and move forward.