The themes of Death of a Salesman include:

A) Willy's quest for the ideal of the American Dream and the idea that those who work the hardest get rewarded the hardest. His idealized notion of the All American "perfect" life with the son in a football team, him a businessman with a wife, even a mistress- and the fantasy of it all.

b) reality vs. fiction- All that Willy had as real was actually his own make-belief notion of grandeur. Back in his time, a salesman was probably the least educated professional of his time, yet, Willy saw himself as a major businessman the way self made millionaires would see themselves today. The dysfuctionality of his family, his lack of parenting skills, his torn marriage, his insipid career, all this goes in the backburner in his mind.

c) fighting against society- A salesman has no choice but to codepend on circumstances: The market, the clients, the trends, the business, etc- Willy tried his entire life to build something he could fall back on with no success. He was as incapable of building a present as he was a future.

d) fighting against oneself- Willy had denied his talents, froze his son's own talents (though Biff was no different than Willy) and all because he still wanted to live this image that he was not up to par. In the end, he died committing suicide, perhaps after finally accepting how little he had accomplished versus how much he had dreamt.

Arthur Miller's Death of a Salesman deals primary with an examination of the human perception of greatness and its repercussions on the American dream. Unlike so many classic tragedies, Willy Loman isn't a man of greatness who falls from on high, he is a "low man" who attempts to project his own greatness and merely drags others around him down as well.

Set against this backdrop are also questions regarding the importance--or perhaps backlash--of modernity. Willy Loman seems obsessed with new gadgets and inventions despite the fact that his own job, that of a traveling salesman, is almost obsolete. Even the Loman house--once idyllic on the outskirts of town--is depicted as being surrounded by tall, modern buildings, as if it is almost being swallowed up by new times and new ways. We also see this idea expressed in the nostalgia and/or regret many of the characters have for the past and their prior accomplishments--both real and imagined. Willy, Biff, Happy, and to a lesser extent even Linda, all suffer from this flaw. 

A prominent theme in Death of a Salesman is the myth of the American Dream. Though Willy Loman has worked hard for decades, he is unable to raise his socioeconomic status. As he closes in on the end of his career, he is, in fact, demoted. He goes from earning a salary to being paid on commission. He is robbed of his dignity by having to grovel unsuccessfully toward a younger boss, and he struggles to meet 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.