Why Communism blows chunks:
A Communist state, as outlined by Marx and Engels, grows out of a dictatorial regime(Socialism) where power is entrusted to an individual, who equalizes the country by stripping the "bourgeousie" of its ownership of property and the means of production and placing those things under state control.
1. So, you've debacled the monarchy and you are well underway seizing land, food, and basically anything else that can be taken and redistibuted under the auspices of furthering the revolution. When, suddenly, people are pissed off that you're taking their shit. They rebel, you slaughter them. Chalk one up for your utopian vision.
2. So, you're finished murdering various pissed off peasants, when, oh shit -- you realize you have a country to run and your band of revolutionary zealots can't hack it when it comes to accounting, or diplomacy, or basically anything other than preaching the Marxist gospel and killing non-believers. So, you set out to build up a bureaucracy in order to support your ambitious goals -- your Great Leap Forwards and 5-Year Plans. Elitist bureaucrats who run the organs of institutionalized oppression. What can go wrong?
3. So, you've institutionalized the revolution. You live in opulence. Your bureaucrats live in opulence. And your people live in utter poverty. Whoops. So, much for a classless society.
4. The niggling thing about Communism (and Socialism by default) is that it requires cooperation. Hence, Communism (community, communal, common, etc) and Socialism (society, associate, etc). People need to buy into your massive land reforms, redistribution campaigns. They need to believe in the ultimate goal -- a classless, stateless society. They need to believe that that goal will eventually be reached. And if it isn't reached, God forbid they would want to rise up from their misery through market reforms, privatization, democracy, and ending your corrupt regime.
Do I hate Communism? No. There have been various instances of small-scale, functioning communist societies around the world. Why do they work? Because people voluntarily enter and leave these communities. They truthfully believe in the mantra: "From each according to his ability, to each according to his need." Communism is a delicate system, much more so than Capitalism, as it requires that everyone share the fruit of their labor with each other, without compensation, just the expectation that the others will do the same. And ultimately, greed isn't the sin that destroys Communism. That's just a simplistic and flawed view of the issue.