Brad DeLong asks, What was Karl Marx's principal contribution? and concludes that it can all be summed up in about ten paragraphs on the philosophy of history.
I will leave to real Marxists (which I am not) to post a strong defense of the guy, but I have to say I'm surprised that someone who claims to take history seriously, as Brad does, would devalue Marx's contribution to that field. Given the evidentiary record available to him at the time, Marx was an extraordinary student of British history during the previous three centuries. Wouldn't it be fair to say that the largest part of modern British historiography is in some sense in dialog with Marx?
Marx's main failing was to generalize from this single case an entire theory of political, social and economic development. The further Marx (or his disciples) strayed from England and the time period of the emergence of capitalism, the worse they have fared. This should not obscure the accomplishment, however.