kalirion: Like I said in my prerequisite, communism would work if human nature was completely different from actual human nature.
DubConqueror: I don't understand human nature. To me, it feels completely natural to take into consideration the common good in everything I do and to work, not for the money, but for what it brings to other people.
What we call "human nature" is usually just the locally naturalized version of culture (that is, the local cultural values, internalized through socialization, and more or less dishonestly described as "natural" and "universal"). So, don't take this notion too seriously.
But still, there are some cognitive (biological, I'd say) limitations to the human's quest for the common good. Due to the fact that it's harder to care for more remote, abstract humans, especially if they are defined as different and other. People sacrifice their own good (to some extent) more easily when the results are concrete, and benefit humans from their own circles. The wider the system, the more people feel unfairly dispossessed in the profit of dubious aliens. And by dubious aliens (the invisible cogs of the redistribution machine).
Seriously, if you're part of a village (and/or family network), with its own redistribution solidarity system, and then some technocrats arrive parachuted from some administrative center, and start diverting your resources to unknown people based on unknown calculation, you might feel robbed. You might resist. And, in the name of super enlightened rationality, these technocrats may exert violence in return.
This is one aspect. "Human nature" (cognitive limitations) being, here, the fact that the (forced) participants in a system don't have access to the whole network of needs and solutions and to the human realities of that rational organisation's requirements. They don't have the global, god-like, overarching vision.
And the other problem is that these technocrats don't have it either, because such global rational vision is generally bullshit aswell (and very centered around needs and duties defined by the specific subculture in which these technocrats are produced). Clunky, dysfunctional, prone to being hijacked by well-positionned assholes (who feel they aren't doing much harm in the grand scheme of things), and being enforced with all the self-righteous violence that this sense of superior rationality seems to legitimize. Not to mention that, at the organisational center of the machinery, decision makers are fed with a little too intoxicating power (over their abstract little rearrangeable and expendable pawns of citizens)...
And even more, imagine this mess in a pre-digital world, where continent-sized organizations run on papers and stamps. Even with modern computers, the bigger the (administrative) machinery, the less it tolerates jiggling cogs. And humans jiggle. They are humans, they have their own lives, their everyday concerns and irreductible freedoms, they make bad cogs. That, more than nature, the human condition. The little room-for-breating required by a human is okay in a short organisational chain. In a huge centralized monster, these tiny individual human elasticities make the whole system shake along its chains and break or clog.
So, tiny causes (human's social horizons, global organisation hubris, individual everyday life agencies), related indeed to what sort of animal the human is, dooms the communist project from the start. Even if it took place in a global culture of benevolence and generosity.
Good intention is not enough. We'd also require, each one of us, a knowledge, and a "sense of the impact upon others", that far exceeds our abilities. Without that, the dedication can only be limited (and the organisation itself flawed).
Just to say, the human aspects that prevent communism to work are not some "evil, selfish nature of man". It cannot really work with good, well-intentionned people either.
