The problem is, Marx - unlike his modern successors, defined capitalism and communism in a very specific way. In Marxism, not every system where people privately own means of production is 'capitalism'. Clearly he differentiated between that and, for example, the slavery-operated economics of the American South, and the economics as practiced by feudal Europe. Equally he did not define 'communism' as an ideology, but as the state of affairs that would be reached as the final stage of history. (This is why Communists claim the USSR was not Communist - it had practiced communism as an ideology, but had failed to actually achieve Communism).
Marxism has multiple philosophical and economic weaknesses. It's prime weakness is that it claim to possess a scientific-like process by which it can predict the major developments of history.
The secondary problem is that it believes that people have inherent interests, derived from their class status, which would lead them to pursue these interests. This would lead to inevitable class struggle. Of course, when men identified with men in other social classes (say, by becoming nationalists, racists, religious or political activists), this was a 'false consciousness' - Marxists know better than you who you should identify with!
The tertiary problem is that of economic calculation. Suppose we have eliminated the motivation problem and established a fair, honest, un-corrupt Communist society - something which has so far not been possibly beyond the scale of a small village, and even there it seems to only last with state subsidies. Without profits and price mechanisms, how will the Communist society know what needs to be produced? Are we making 9,000,000 pairs of blue jeans and 1,000,000 pairs of black jeans this year, or vice versa? Regular condoms or blueberry-flavored ones?
Then there's that pesky motivation issue.
