The main point of this book is that capitalism, and the inequality that it brings, is unjust and that a just society would do away with it. When Julian West wakes up in they year 2000, he discovers that a just society has arisen in the time that he slept. This society has done away with the evils of capitalism, greed, and inequality.
In the book, Bellamy is arguing that big businesses have to be reined in. He argues that they do not exist to help society. Therefore, he foresees a future in which the companies have all been nationalized and are owned by the government. He does not use the words "socialism" or "communism" but this is clearly what he is talking about. He believes that businesses owned by the government will operate for the common good and not simply to enrich their owners.
Therefore, the major point of the book is that capitalism is based on greed and creates inequality. This leads to an unjust society.