One thing that needs to be said, is that although our world is filled with examples to the contrary, on paper, communism does not necessarily preclude democracy. Although those who have adopted it have gone on to make an utter mess of themselves, if properly executed, it is as viable a system as any.
Communism has this negative connotation in our common parlance, and it primarily stems from the way in which many communist societies have fallen to the wayside in overall prosperity. While it is easy to blame the system, it is infinitely more accurate to blame the circumstances. The Soviet Union, for example, was drawn into a long Cold War against an enemy that was simply capable of out-producing, out-purchasing, and all around out-fighting it. It went broke fighting a war that it had no business fighting.
Communism has also, repeatedly, been a vessel by which a few powerful can rule over the many poor. It is identical to capitalism in this regard, but capitalism seemingly has a better publicist.
In a communist society, if you are poor and your neighbour is poor, you are destined for a life of poverty.
In a capitalist society, if you are poor and your neighbour is poor, you are also destined for a life of poverty - but you're lead to believe things can be different. And while they can, that same astronomically unlikely chance is the same chance a person in a communist country has. Just like you can strike it rich becoming a movie star, or oil tycoon in a capitalist society, you can beat the odds and become a party big shot in a modern-day communist society.
The overall problem is no matter which way you go, our hearts and minds are still very much obsessed with stuff. One system promotes this obsession while the other abhors it. The difference being, the one that purportedly abhors it secretly obsesses with it too, because that's just how human beings are in this day and age. And it works against it. Capitalism is meant to reward greed, but communism is meant to stop greed. Ergo, if too many people get greedy in a communist society, the entire system goes to shit.
If we look at the Star Trek universe – in particular, the Federation – we see a quasi-communist society that works rather well.
Unfortunately, the truth behind that, is that with technological advances that create food out of air, that society has no 'have nots' - at least, not like ours does now.
And the sad truth of it is, until we do come up with that sort of technology, the system that favours the greedy will be the system that works the best…