Oh the deadly homogenous label of "human nature"...
I've outlined my views on this simplistic view of humanity in another thread. But at the base, communism is probably the most morally perfect governmental system out there. It is far from evil, and far from the LEAST practical political system ever thought up. However, it is unworkable... Even in small tight-knit groups. It may last for a year, maybe 10 years, but the problem is that once a society is invaded by a single successful exploiter, the system falls apart. Communism can never compete with capitalism because a communist nation will have it's work cut out for it getting rid of all exploitative persons as well as keeping them out. Especially with all these greedy capitalists around.
Not to say that capitalism is bad... I very much admire it. It has some of the same problems in that for it to be ethical, it requires a homogenous population as well... Unfortunately, not everyone is created equal, and there is much stratification. I don't buy the argument that since there is inequality in nature, then obviously we shouldn't be alarmed at inequality in society. If humanity has any gift over nature, it's the power to deviate from it using our ingenuity and technology, so that we may shape our environment, rather than be shaped by it.
I think the largest misunderstanding regarding communism is that it's an economic model, not a political one. In that, it's opposite is capitalism, not democracy. You can have a democratic communism in theory. In fact, the labels are fairly arbitrary, since communist principles can be found occurring naturally in capitalist societies. For example, the corporation, perhaps the flagship for capitalism, is fairly communist in nature, in that it's a large group of people pooling together their wealth for their mutual benefit. Of course, it is still guided by capitalist economic theory.
Most people cite work ethic, or the lack thereof, as the major downfall of communism... but that's definitely only in view of our modern Occidental values. Humans are not necessarily greedy and lazy. It really depends on the culture. Certain cultures place community over self, and wouldn't worry about whether or not someone else is working less, but rather appreciate their own contribution to society. There are other motivations to work besides money...
Of course, in the end... communism requires that everyone set aside their anachronistic natural instinct and choose the more abstract moral values. It's a dog-eat-dog world, but we aren't dogs. Unfortunately... for such a change to happen, the entire individualistic trend will have to be reversed, the only feasible way being a massive global propaganda campaign aimed at re-adjusting social values (as much as I hate to say it); so large that it would make the current anti-smoking propaganda in the western world look meek.
I suppose I've proved the point that communist idealists usually end up advocating some large-scale revolution... involving propaganda or forceful takeovers, or whatnot. But it's not like capitalist idealists (aka free-market capitalists, of which I am one) are any freer of such notions.