On reading pages 602-635 of Grundrisse, I found the following points worth emphasizing:
(1) Critique of Malthus' theory of population (which even Ricardo had used): the two main points of Marx's critique being (1) theory of population is not ahistorical as Malthus thinks, every mode of production has its own theory of population, and (2) Malthus' theory implicitly assumes full employment of labour, an assumption that is now ubiquitous in neoclassical economics (here again, the point is to realize that the relationship between the means of subsistence and population is mediated through the relations of production; it is not direct as Malthus assumes). Interesting aside: Malthus's theory is not his own! He borrowed a lot from James Steuart, I think.
(2) Surplus population or the industrial reserve army of labour: the mains idea here, to my mind, which Marx developed more fully in Chapter 25, Vol 1 of Capital, is that capitalism, as a mode of production always needs a substantial body of surplus population of workers (the industrial reserve army of labour) for its stability and continued existence; the process of capital accumulation, in fact, produces this surplus population over and over again through the displacement of labour by machinery. Since the unemployed are only a small part of the reserve army of labour, the study of the latter should replace the more restricted study of unemployment; this is one of the differences, to my mind, between a Keynesian and a Marxian approach to the study of labour underutilization.
(3) Critique of Smith?s theory of "labour as sacrifice": I found this point absolutely fascinating; when one grasps the positive, liberating aspect of labour, as Marx emphasizes, one is immediately able to see the idiocy of all of modern neoclassical macroeconomics that is based on the so-called labour/leisure choice.
(4) There is a nice discussion pertaining to the turnover time of capital (as the sum of production time and circulation time); the example where the annual rate of profit increases because of the increase in the turnover time of capital is illuminating.
(5) Cost of circulation as deductions from surplus value: the important point here is that exchange does not create value and hence the part of total social (labour) time that is devoted to circulation is a cost from the point of view of society, is a deduction from social surplus value. (Important exception: Transportation of goods from one location to another is not to be considered as circulation; it is a part of production and adds value. This has a nice contemporary ring: state contingent commodity markets!)