The French guillotines and the communist labor camps and killing fields and forced famines were pretty good at eliminating people who would not practice equality, who would not give up some of what they had in order to even things out, who would not give according to their ability and take only an equal share, yet no permanent adoption of Marx's ideas resulted, so maybe it is a law of nature rather than a human perception of value that makes Marx's ideas unworkable.
As others have stated, human nature is why Marx's ideas, which look great on paper, will never work in practice. Even though we profess to admire the idea of equality, when all is said and done what we really value is the ability to get not just what we need but what we want as well. We want to be paid in accordance to what we feel we are worth. The idea of a society in which everyone gives according to his or her ability and receives an equal share of the wealth is great, and if it could be implemented we do have enough resources for all, but the problem is that this would mean that the rich would have to give up some of what they already have in order to even things out. That is not likely to happen unless human perceptions of value change radically to shift away from goods and possessions, the foundation of capitalism and the driving force of our global society.
Certainly, Marx's ideas about Communism and Socialism have been addressed in the previous posts. Their thoughts are accurate. However, I think that the time in which Marx is writing is one where individuals placed full faith in the capitalist system, a form of economics that was unchecked and unregulated by governing bodies. Marx's writing certainly might be articulating a vision of its opposite. Yet, his writing might also be calling attention to the fact that nothing can exist in its purest state. Just as Marx's own vision of socialism or communism can never exist, perhaps the same can and should be said of a capitalist entity. With this in mind, "Marx's ideas" might be seen as more of a critique of capitalism, a call for it to change. In this light, Marx would concede that while more should be done, the modern understanding of capitalism as one that needs change and has undergone reform would be something that might actually be a lasting legacy of Marx and his ideas.
I agree with above post. For communism to work on Marx's model, humans would have to be basically altruistic - to give to the society as a whole with no immediate benefit to the individual, and to do so all the time, with everything a person makes. It's not that humans can't be altruistic - they can be and are - take donations for Haiti relief for example. But altruism is the exception in human nature, not the norm.
Human nature is competition. Human nature is incentive-based. Even evolution is competition. Humans are hardwired to try and improve their lot in life and that of their children. We are not hardwired to give everything without personal benefit. The person that can and does do that is the exception - priest, social worker, etc.
Stalin, Castro, Kim Jong-il - these are all communist leaders who were extremely personally wealthy. Even in the countries that espoused communism, the leadership never practiced it.
For me, the reason why Marx's ideas will never happen is because he does not seem to truly understand human nature. I think he takes too positive of a view of human nature.
In the Communist Manifesto, Marx seems to think that people will work simply because they should do so. He thinks that each person will give according to his ability even when he has no real incentive to do so.
I am sure that there are people who will work very hard even if they cannot hope to get rich or even to get anything from working that they could not get just for sitting around. But there are a lot of people who will not be motivated to work (especially if their jobs aren't wonderful) unless they can get paid.
If people were angels, Marx's ideas might work. But I believe that we are fundamentally pretty greedy and are not likely to work our hardest unless there is something in it for us.