The whole Tyrone family is in a state of despair, and it's hard to think of an author better at capturing despair than O'Neill (in no small part, one suspects, because he came of age in the sort of environment depicted in this play). O'Neill was certainly bitter about his past, but, importantly, he doesn't lose perspective. Although the way the Tyrones treat each other ranges from neutral to downright cruel, O'Neill does a splendid job of balancing this against the fact that they all love each other deeply and feel very unnerved whenever they realize that they're treating each other unfairly. Despite all the problems he faced as a young adult, O'Neill always viewed his family with a good deal of love and reverence, and that comes through in the play. As Mary puts it, "None of us can help the things life has done to us. They're done before you realize it, and once they're done they make you do other things until at last everything comes between you and what you'd like to be, and you've lost your true self forever." The tragedy of Long Day's Journey into Night lies in the fact that these great individuals have lost their true selves due to the various demons that haunt their lives.
Some of O'Neill's works could reasonably be criticized for featuring relatively one-dimensional characters and formulaic plots. In the case of Long Day's Journey, though, because O'Neill was able to rely on his own experiences, all four main characters are exceptionally deep and balanced, and the plot is distinctly unpredictable. Though I've very much enjoyed all the O'Neill plays I've read, it seems that in Long Day's Journey he finally put together all his talents and produced his crowning achievement.