I concur with #4 above in thinking that actually, although Marx is most famous for The Communist Manifesto, his most valuable intellectual achievements and offerings actually emerged after this time. Perhaps it might be helpful for us to see this work as the match that enflamed the intellectual fecundity of Marx's ideas and thinking, and #4 lists a number of these key ideas.
Marx's thinking became more systematic and almost scientific, as reflected in Das Kapital, which is more of a economic analysis/critique of capitalism than a revolutionary manifesto. He persisted in believing in the inevitability of class revolution, and accepted, even if some other radicals like Mikhail Bakhunin, did not, that a state apparatus might be necessary for a revolution to proceed. It is his post 1848 writings where most of his most important contributions emerge- the his labor theory of value, surplus and alienation, false consciousness, base and superstructure, etc. All of these important intellectual achievements emerge after 1848.
Picture the 19th century, and you can see why someone would be concerned. Industrialism changed society so much in such a short time. Although many changes were positive, quite a few were not. That includes social, economic and environmental changes.
One way that this happened is that they stopped (or at least Marx did -- I don't know how much of his thought was the same as Engels' thought) thinking in such unilinear terms. In other words, Marx stopped thinking that all of human history, in all times and places, proceeded in the way that he outlined in the Manifesto.
Instead, Marx came to believe that different places could progress at different paces. He believed that Russia, for instance, could move from a relatively feudal system to socialism. This sort of a move would bypass the capitalist mode of production.