Well, with any great work of literature such as this play and others like it by Arthur Miller there are a multiplicity of themes that could be argued to be the "major theme." In a sense you might want to ask this question on the discussion board to gain more of a range of responses. However, 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, to me at least, is one of the dominating themes in this excellent play. Good luck with thinking through some of the others!