I think that what I am about to say has already been said in some of the posts above - but I suppose it can't help to say it in a slightly different way (and if I get it wrong, please feel free to shoot me down in flames!).

My (shaky) understanding of Marxism is that it is an evolutionary theory of human society. Said societies go through/will go through various stages, and the ultimate stage is socialism. The various stages include: hunter gathering, primitive agrarianism, feudalism and industrial capitalism. To reach the ultimate socialistic stage a society must go through the industrial capitalist stage and possess a fully fledged working class. This working class will be in the vanguard of the progression to socialism.
The trouble with Russia, at the time of the October Revolution, was that it only had a tiny working class and the vast majority of the population were peasants. The new leaders of the new Soviet Union had to 'shoe-horn' an unready and unwilling population into this new type of society. As the first half of the twentieth century progressed this was done with increasing brutality but because Russians were used to brutality, and were used to being brutalised, they went along with it. Eventually the whole thing proved to be unsustainable.
My, decidedly unMarxist (and probably reactionary) view is that all political/economic ideologies are dangerous oversimplifications and tend to lead to death and suffering (for someone, somewhere) and are all, ultimately, unsustainable. Down with ideological world views, I say!