I suspect John Rawls taught a better course in History of Thought than I do

Well, mine is Economic Thought and his was Political Thought, but there's a lot of overlap. Mark Thoma posts some class notes from a lecture Rawls gave on Marx's theory of economic justice (or lack thereof) at Harvard in 1973.

Summing up. (1) Marx views the notion of justice as a virtue of legal forms and institutions, and thus perhaps it is a notion which belongs to prehistory. The state depends upon the mode of production. (2) Marx doesn't deny that the various conceptions of justice have formal features in common -- exchange of equivalents for equivalents -- but the notion of what is equivalent is determined in different ways. Marx would be prepared to admit that capitalism in its high period is just. One reason he rejects the utopian's argument is that it is misleading. It rests on a misapprehension of where the essential problem lies: not in the superstructure, but in the mode of production. He felt that the key enterprise is to give a scientific theory of the mode of production.

Worth reading in its entirety, especially if you're taking my History of Thought class.