Namely, his starting point is that labor is identified with value. Wealth is created when raw materials and the means of production are combined with labor. In the capitalist theory of value in contrast, one's legal right (inherited or otherwise) is one of the things which is identified with value.
It's not a claim that this value is something metaphysical, just a political assertion that this is what we ought to think of as "value". It's all about what you want to consider as the primary "cost" in production.
This has been refined over the years. I'm not an expert so I may not be able to respond to any and all criticisms but I will try my best.