Communism seems like a good idea in theory. However human instinct comes into play.
Same can be said of literally every governmental or economic structure. They're all based on everyone adhering to some idealized psychology that always has exceptions. Capitalism is based on the ideas that people will act rationally, in their own self-interest. This is great for selfish bastards, and selfish bastards tend to rise to the top in that system. Look at Washington and Wall Street, and tell me whether you think it's a good plan to put selfish bastards in charge.