Answer: 1) one state party that monopolizes all the power (there is no pluralism within society), 2) strong indoctrination in all parts of society and organized by the ministry of information/culture/propaganda (and its apparatus), 3) cenzorship, 4) repressive apparatus, 5) society is "closed" (meaning of "closed society": Karl R. Popper), 6) there must be some official myth turned both to the past and future
Explanation: my answer has to do with the political practice and not with the theory. In fact there were no Communist societies/regimes in this world. There were just 1) Communist philosophies, 2) Communist parties, 3) official Communist propaganda. In spite of all that there are still Communist parties and Communists. Perhaps there are even Marxist historians. Communism has to do with a belief we humans usually have (also thanks to Enlightenment): that each idea can be put into practice. Communist experiment can be viewed as a useful but not successful attempt to put Communist ideas into practice.
