Pure Communism concerns me in a few ways:
1) Compensation. Some jobs require years of learning and training in order for max efficiency. Just take a doctor, for instance. He or she has to spend years learning about the body, equipment, scientific advancements, and procedures before being certified for practice. How do you compensate them for all their hard work in a system that rewards everyone equally or only gives them enough to function? Are they simply expected to work this hard, say, harder than someone who does clerical work, like office administration, fax, file, copy, answer phones, etc.?
2) Assignment. In any event that there is no compensation for more demanding types of labor, who the hell would take on such back-breaking work? How will work be distributed? Will specialization exist in a complete working class, and how will efficiency be monitored?
3) Enforcement. Gatheringsin already brought this up. In ideal communism, there is no need for enforcement - but in any event someone does do something wrong, how are they punished? By the whole 'State'? Even in in a version of primitive communism, which references Native American tribes, there might not be a completely authoritarian rule, but many tribes have chiefs or people who decide the rules, ethics, and moral code. Law does not work in an ideal model of communism; law already undermines the natural disposition of communists to do what is best for everyone. Without law and without enforcers, there are no punishments for deviants.
4) The Greater Good. Communism doesn't necessarily define what the 'greater good' actually is. It has conditions for the greater good: absence of authoritarian rule, equality, work and function with only enough pay to survive, no private property. To this end, these conditions produce slaves with no masters.