the basic expenses of maintaining his home.

Another theme in the play has to do with aging. Willy is not only past his prime earning years, he is also beginning to become mentally impaired. He is no longer able to drive safely, and he begins to hallucinate (he frequently sees his late brother). He reflects back on his former sexual virility when he had both a wife and a mistress. He is not of much use to his adult sons. Age has taken away many of the ways that Willy Loman defined himself as a man.

There are a number of themes explored in Death of a Salesman.

The theme of honesty and delusion is thoroughly explored and is central to the play. Willie feels that he needs to be more successful than he has been. He is unwilling to face the truth of his position, of his relationship to Biff, and to accept his declining abilities. Willie's denial is one example of the cross-section of delusion and dishonesty presented in the play.

Resilience and compassion are also themes. Linda and Biff are the characters who most clearly demonstrate ideas of caring and of the difficulty in caring for family in times of strife and psychological difficulty. Both Biff and Linda put themselves aside at times to help Willie and to try to fix things.

You can find more on themes in Death of a Salesman at this eNotes page: http://www.enotes.com/death-of-a-salesman/themes.

With any great work of literature such as this play there are a multiplicity of themes that could be argued to be the "major theme." Just to give you one theme that dominates this work, I will talk about the commentary this play gives on Capitalism and the Value of Life.

What is truly tragic about the story of Willy Loman is that he arrives at the conclusion that he can only save his life by losing it. He believes that committing suicide is the only way he can redeem himself in his own eyes and gain some tangible benefit for his family. The play raises the unpleasant notion that tragedy may befall the most ordinary of life (even the "low man") in our society today, and for this reason it throws up massive issues about the way we all live and work and dream of happiness.

For us who live in a society that is dominated by capitalism and who believe that happiness is based on the accumulation of wealth, Willy Loman in a sense presents the ultimate challenge to an 'unreal' society which is based on capitalism, since he concludes that $20,000 is worth more than his life. And yet the audience is left asking the question if a man can really be valued at the amount of money he is worth. If this is the case, it is hard to escape the conclusion that capitalist societies such as the United States have reduced human beings to commodities, and dehumanisation is inevitable.

This play also deals with appearance vs. reality.  Biff "appears" to be the model of successful American boy.  He is athletic and well-liked.  Bernard is studious and quiet.  However, it is Bernard who is both happy and successful in life, while Biff struggles to find his place.

Linda appears to be a supportive and caring wife, but there is also the opinion that she is an enabler.  Rather than confronting her husband about his disillusionment, and helping him to face his failure and move on, she encourages the boys to help her indulge Willy in more fantasies, more elaborate "plans".

This inability to see things clearly are keys to Willy's suicide and Biff's unhappiness.

Another theme I would like to add which is linked to the other answer is the profound message that this important text gives us about the value of human life. Of course, the play presents us with a society that has sold itself to capitalism and the American Dream, yet the ultimate challenge of Willy Loman to such a society is his conclusion that twenty thousand dollars on balance is worth more than his existence. The text asks us some very hard questions that we are not able to ignore, such as can a man really be only valued at the amount of money he is worth? The disturbing suggestion of this text is that societies like America that have embraced capitalism wholeheartedly have as a by-product reduced human beings to mere commodities, with a resulting dehumanisation that we see in the character of Willy Loman.

There are several themes to Miller's drama.  I think that the most compelling of them is the critique of the American Dream.  The idea that emerges throughout American History, and especially true in the 1950s, is that the "American Dream" is defined by monetary success and the trappings of wealth.  The natural connection was that emotional and domestic happiness will follow material wealth.  Somehow, if individuals worked hard and made a great deal of money, they would be happy.  In seeing the trials and difficulties of Willy, the reader understands that this is not the case.  In this light, there is a strong grasp of how there are other dimensions and components to achieving happiness in consciousness.  No matter how much Willy works, there is a hollowness present, an emptiness that cannot be avoided.  What Miller himself would term, "the matrix of his life," is one where the pursuit of material wealth is unable to fully remedy the emotional pain that exists in his life.  In this light, Miller's drama explores the fully complexity and richness in his depiction of the American Dream.

Perhaps the most important point Arthur Miller makes in Death of a Salesman concerns the false and corrosive nature of what is sometimes called the "gospel of success." This is an idea based on the works of various nineteenth-century writers, notably Horatio Alger and the multi-millionaire Andrew Carnegie, who encouraged the idea that there was no limit to the wealth and success that ordinary Americans could achieve with hard work and perseverance. This belief in the possibility of economic success is at the heart of the American dream.

Willy Loman is an ardent believer in the gospel of success. He admires wealth for its own sake and has an idealized and deluded image of himself as an outstanding salesman who makes large amounts of money through his popularity and charisma. This delusion extends to his family, and he makes Biff miserable by insisting that he, too, measure his personal worth in terms of financial and professional success.

Willy is miserable because he is a failure on his own terms. Biff is miserable because he is a failure on the terms his father insists are the correct ones. Willy's addiction to the values of corporate America ultimately harms himself and everyone around him, and the play shows that the type of success Willy wants so desperately is not worth having. The wealthy characters are selfish, egotistical stereotypes, and despite his embrace of these same superficial values, Willy himself is neither powerful nor successful. If he could give up the gospel of success, Willy could be a good husband and father, since his wife and sons love him anyway, but he fails to appreciate what he has already.

Death of a Salesman is a play about Willy Loman, a none too successful salesman,  his wife, Linda, and their adult sons, Biff and Happy and the difficulties within their family.  Willy and Biff do not get along at all.  Most of the action in the play occurs in the form of flashbacks with Willy displaying some emotional instability, having tried on several occasions to commit suicide. 

In Act II Willy loses his job; he is offered a job by his brother Charley, which he is too proud to accept; however, he continues to accept loans from him. In a flashback we learn that Biff had caught his father in a hotel with another woman and become so disillusioned with Willy and life’s possibilities that he gave up an athletic scholarship and a college education.  While his sons are home, Willy becomes more and more unstable, and in the end, speeds off in his car.  The audience next sees the family at Willy’s poorly attended funeral.  A more though explanation and summary of the play can be found at the link  below.

There are a number of lesser themes woven into Miller's fine play, but the core theme is what happens when a dream—especially the American dream—dies. Willie Loman wants greatness as intensely as anyone who's ever made it big in America ever did. However, he simply doesn't make it, and all the other themes of the play—the meaning of aging, the nature of family love, etc.—have to be made sense of in relation to this core theme. What does it mean in a land where anyone can be great…to be nobody?

Greg

One of the major themes in Death of a Salesman is hard work and the American dream: the idea that anyone in America, regardless of background, can find success. Willy, the play's protagonist, has a somewhat naive belief that the American dream is really about charm; a man in business should be well-liked and charismatic in order to find success. In reality, the American dream actually takes hard work, and Willy doesn't seem to realize this. For example, he is inspired by the success of both his brother Ben and his friend Charley's son Bernard. Both men found success, but not from being likable and charismatic. Ben travels through Africa and Alaska and ends up finding a diamond mine, creating huge success. Bernard, who is studious and shy as a child, works hard to get into and get through law school and becomes a successful attorney. All the while, Willy is so concerned about the financial and material aspects of the American dream that he forgets to pay any attention to the good things around him, such as friendship and the love of his family. At the end of the play, Willy commits suicide, hoping his wife and sons will get his life insurance proceeds; he literally ends his life for money. Miller seems to be trying to tell a cautionary tale about the American dream: it is important to work hard to find success for yourself, but there is a fine line between hard work and turning from a human being into a financial commodity.

I think that Willy's narrative holds much in way of personal revelation.  Willy defines himself, his reality, and his sense of success through external or monetary terms.  This is where his fundamental failure lies.  In any configuration, if happiness is defined externally, there is a greater chance for pain and heartache than spiritual redemption.  Willy is crushed under the weight of his own dreams because of the absorption of the world around him as one that defines success through terms of money.  I would say that this is one of the profound revelations of the drama.  In defining oneself externally, pain and suffering is evident.  Miller himself sees this when he saw others view Willy's predicament:

[Audience members] were weeping because the central matrix of this play is ... what most people are up against in their lives.... they were seeing themselves, not because Willy is a salesman, but the situation in which he stood and to which he was reacting, and which was reacting against him, was probably the central situation of contemporary civilization. It is that we are struggling with forces that are far greater than we can handle, with no equipment to make anything mean anything.

It is this statement that ends up defining so much of what this play reflects about ourselves.  Willy represents what it means to not have any spiritual or internal resource to guide us, the essence of what it means to have "no equipment to make anything mean anything."  This lack of meaning is a brutal end for human consciousness.  Willy ends up teaching the reader and the audience that to lack this capacity is a recipe for personal disaster for ourselves and those who have the curse of loving us.  If we wish to have any happiness in our lives and wish to bring happiness for those who happen to love us, there has to be a struggle, a striving, to explore a personal code of conduct where meaning is internal, enabling us to possess this "equipment" where happiness is not externally driven.

Death of a Salesman emerges out of the grim circumstances of the Great Depression, which Miller experienced as a teenager. His own family suffered greatly during this time: his father lost his business, and the family was forced to move to Brooklyn, which is where, in the play, the Lomans live. However, in his autobiography, called Timebends, Miller said Willy Loman was based not on his father, but on his uncle, a man named Manny Newman. Miller wrote that Newman was ʺa competitor at all times, in all things" and always clung to the optimism of the American Dream and the "shouts of victories that had not yet taken place but surely would tomorrow." (These words echo Nick Carraway's words about another American dreamer, Jay Gatsby, in The Great Gatsby.)

We can see the close similarity between the names Willy and Manny and Loman and Newman. Like Newman, Willy is highly competitive, which works against him, as he sees other people as enemies he wants to best and beat rather than friends. Also like Newman, Willy was a failed salesman barely hanging on but ever hopeful.

The play was written in the late 1940s, when prosperity was on the rise after the end of World War II, but it also reflects the lingering mindset of the 1930s, a period that scarred many people. Rather than focusing on the play being based on a particular real person or event, however, it is best to understand it as reflecting what has happened to many Americans who have seen their hopes of prosperity and success dashed. Miller meant for his play to have a universal appeal, and this is evident in the details—or lack thereof—in the play itself. For example, we never learn what Willy sells, as Miller does not want people to focus on a particular business or industry but rather on the idea that times were hard across the board for anyone trying to sell to make a living.