Karl Marx is the most overrated socialist writer ever. There’s virtually nothing in Marxism that is original. The ideas for which he is best remembered are not original at all. The “theory of surplus value” was espoused by Pierre-Joseph Proudhon before Marx ever dealt with the subject. The concept of the “immiseration of the proletariat” was espoused by Louis Blanc. The model of State Socialism, where government takes over and runs industries, was also an idea espoused by Blanc. Blanc also made the observation that capitalistic production tends to create disequilibrium of supply and demand, leading to crises of overproduction or underconsumption and that this tendency causes capitalistic nations to need an ever-expanding market. The notion of the “withering away of the State” and the ‘governing of men giving way to the administration of things’ was espoused by Henri de Saint-Simon. The idea that a relatively small clique or vanguard should conspire to take over the State and usher in the dictatorship of the proletariat is eerily similar to Louis Auguste Blanqui’s proposal. If Marx was original at all, it was merely in combining various competing socialisms into a new type of socialism. Unfortunately, he tended to latch on to the most authoritarian and anti-democratic strains of socialism.
Marx also introduced economism and Hegelian religious nonsense into his socialism. Large parts of “Capital” read like something that could have been written by Carl Menger or Murray Rothbard. Marx gets overly caught up in economistic drivel and pedantic nonsense, which consequently makes reading certain of his writings utterly dreadful. The worst part is that Marx latches most firmly onto classical economics in the places where it is most painfully incorrect. The “labor theory of value” is dwelt on forever, when the gist of what Marx was trying to say could have been said better if he would have thrown out the labor theory of value and drawn a simple distinction between the cost of production and the price at which the final product is sold, while insisting that labor produces wealth. In fact, this is an approach taken by some later Marxist writers.
Then there is the fact that orthodox Marxism really is a defunct and obsolete theory. The message of “The Communist Manifesto” is essentially wrong. The immiseration of the proletariat hasn’t taken place, because social democrats have been able to push through ameliorative reforms that kept the proletariat from becoming so impoverished that it had no choice but to rebel. Minimum wage laws, laws regarding safe working conditions, and other reforms have saved the workers from becoming so immiserated/impoverished, and so the workers do not feel a huge need to rebel and overturn the existing system. Eduard Bernstein’s critique of orthodox Marxism and revision of Marxian ideas along “evolutionary socialist” lines makes much more sense, as do the ideas of the Fabians. And there is much to be said for Murray Bookchin's notion that the revolutionary class of tomorrow will not be the working class, but rather the people who can't find work. Guy Standing's idea of the precariat replacing the proletariat is a valid observation. And the fact that Marx clung so dearly to Hegel that he ended up espousing a sort of “Calvinism without God,” turning Marxism into a quasi-religious ideology, really doesn’t help his case. The pedantic dialectical approach, which has its value and should not be abandoned, was turned into a sort of religious dogma, rather than a mere analytical tool. His Hegelian ideology led him to espouse a most unscientific socialism. Marx called his theory “scientific socialism,” not because he followed the scientific method, but merely because it was dialectical and materialist. Dialectics is not scientific as much as practical. Materialism is a metaphysical creed, not science.
Marx is still interesting and perhaps worth reading. His analysis of the "primitive accumulation of capital" is on point, in spite of Peter Kropotkin's valid critique of it. The problem with Marx is that he overshadows all the other socialist writers, so that people think "Marxism" whenever they hear "socialism." The result is that other socialist theorists, who are more libertarian and more scientific, are forgotten and ignored. If you really want a good understanding of socialism, you also need to read Peter Kropotkin, Eduard Bernstein, George Bernard Shaw, and Abba Lerner, all of whom had much more democratic and libertarian approaches than Marx did.