First, let's make it clear that as soon as you talk about socialism, it must be authoritarian (though of course, how authoritarian varies). Socialism cannot ever work without the state, and the more rules it applies, the more authoritarian it must be - even if the authority comes from democratic voting (remember the good old "two wolves and a sheep voting about what to have for lunch"). In the purest economic sense, socialism is a system where the means of production are owned by the state. But even limited socialism (as is rather popular nowadays) needs to use violence to force people to do things they do not want to do, and that directly harm them. Every price control, every regulation, every subsidy, every trade union etc. rely on (usually threat of) violence to be implemented. You may agree or disagree about whether a particular socialist law is good or bad, but regardless of that, ultimately it has to be implemented by force. But let's not get too deep into that.
If you exclude socialist communism (the kind that's actually been implemented in Venezuela, Cuba, the Eastern Bloc etc. etc. etc.), you're left with a simpler idea. The means of production are owned by the community. Now, this is a state that existed for quite a long time on Earth. Tribal economies are usually communist. You have your axe, but the grain stores are communal. You have your bow, but what you hunt is shared among the people. You have your clothes, but... you get the idea. Private property exists, but it comes from "grants" from the community - you do not automatically own the leather you get from killing a deer.
Is this what Marx had in mind? Hell no. Marx had an idea of an inevitable progress from primitive economies to advanced economies. In his view, such "tribal communism" was primitive; feudalism was more advanced, then followed capitalism and finally communism. He expected that it is the most advanced capitalist economies that would result in communism (and wasn't very happy that communism was ever only adopted by the most backward backwater countries in the world, starting with Russia of the time).
How do we get to the authoritarian problem in communism? The central problem of economics is this: some resources are scarce. How do we best allocate these resources in a way that satisfies the most people (or the ruling class, or the priest class, or ...)? And communism's answer to this is essentially "...". In a way, it claims that by the point the capitalist order changes into communism (voluntarily!), this becomes a moot point; the value of a thing will be solely decided by the effort put into making that thing. You might be thinking I'm setting up a strawman here, given how ludicrously naive and simplistic that is, but this is the actual Marxist theory of value! Of course, even Marx couldn't ignore that this is completely wrong, and his solution is well known to every high-school physics student - if the numbers don't fit, introduce a fudge constant. The price of every item is the amount of effort expended, plus a traditional overvalue (different for each product).
Tribal communism works fine, mostly. But it severely limits the opportunities for specialisation, and division of labour. Our entire economic world cannot survive without unbelievable amount of division of labour, which comes entirely naturally, without any conscious direction or state intervention, without any central planning - thanks to free-market capitalism (capitalism didn't even need a name before the socialists came - it's just what humans do if you don't put too many barriers in their way). Of course, when talking about capitalism, you're bound to encounter many capitalism strawmen, both from the opponents and the advocates. Well worn phrases like "people only do things for their personal benefit" are either tautological or nonsensical, depending on how you define "personal benefit", but I digress. The point is, division of labour is why the planet now hosts billions of humans, instead of hundreds of thousands. It's why after a relatively brief dip in living quality in the early days of agriculture, we've been on a steadily rising incline; of course, it's not just monotone growth - there are crests and valleys. But ask a starving kid in one of the places less blessed with free-market capitalism whether he would gladly trade his place with you :)
We can't go back to small scale communism. Of course, you can still form small communities, even limit trade with the rest of the world quite a bit (somewhat like the Amish do - they're not truly isolated, but still get quite a bit of autonomy from the world economy). But you can't do that to the whole world population. The only result of such a thing would be the largest genocide in the history of mankind. You need a way to keep division of labour going. And that's a bit of a problem, because tribal communism depends a lot on how we evolved. When you live in a tribe, there's a certain degree of relatedness between the people. Now, you might have been taught that natural selection benefits individuals that are better at surviving, reproducing etc. But that's subtly wrong - it's selection over genes, not individuals. If you sacrifice your life to save your ten full brothers, it's a loss to you as an individual, but a wonderful trade-off for your genes; on average, each of those brothers shares half of your genes (that is, shares 50% of the variation in respect to a random individual in the species). On average, each of your genes lost one copy of itself, but maintained five copies that would otherwise be lost. Tribes naturally get altruism, merely from biological natural selection, even in the absence of culture.
Of course, you don't have that luxury with the whole world's population. One of the tenets of socialist thought is that human society is consciously shaped by humans; that we're smart enough to realize that cooperation is to our benefit, and act upon that. The natural communist follow up is that all you need is teach people that cooperation is a good thing, and they'll keep cooperating even though they could get bigger benefits from not cooperating. But this is not what you'd call an evolutionary stable strategy; any individual that defects, just a tiny little bit, has a lot to gain. He doesn't even think of himself as bad; you can always rationalize that tiny little bit of defection (human brains are very good at that, since it's one of the big parts of our political instinct, which is a very strong force in the evolution of humans). He believes he's doing the right thing. All the same, the economy of the whole society goes down (just a little bit). Of course, the harm to each individual is smaller than the benefit to the defector. But if enough people defect, even just a tiny little bit, eventually there comes a point where the harm to each individual is larger than the benefit to each of the defectors. Even on the individual level, everyone is hurt by the defection.
But there's no correcting force. The society will decay if left to its devices. Even though the end result is harm for everyone, every individual faced with a decision between cooperate and defect will benefit from the defection. Of course, what most often actually happened in the real world was one of three things - either the communities limited their size and trading with the outside world (i.e. some form of tribal communism), or they adopted private property in the wider sense of free-market capitalism, or they adopted kings - the authoritarian approach. Even a very poor authority can be preferable to a world with no recognition of property. Marx saw communism as more advanced than capitalism; but in reality, it was a return to the complete anarchy of humanity's earlier times.
That's really the crucial social problem with communism. If you could actually educate (or breed; both have been tried by would-be socialists and communists in the past) humans to prefer the long term benefits of a freely cooperating society even though in the short term you could be better off defecting (a tiny little bit) - you could build communism without authoritarianism. It goes contrary to natural selection on genes, but it might be possible to create a cultural environment in which natural selection on memes will overpower that (just like it allowed use to develop contraception and not be replaced by faster breeding stock of humans... yet).
Unfortunately, that's still not the whole story. Maybe we could make a society that has enough of a preference for freedom and cooperation to make communism work like that without violence, even though it's rather unlikely to say the least. But we also need to go back to the theory of value. You need something that tells you how to use those scarce resources. Even if we had a community of people who were very happy about sharing, you still need to decide whether a new winter home should be built in place A or place B. Human labour is still a scarce resource. Land is a scarce resource. Building materials may be very cheap, but ultimately they're still scarce (even if we develop materials that are essentially free, like plastic, you still have cases where a scarcer material would be cheaper in total - e.g. we can't build turbines from plastics yet).
I don't think anybody ever compiled a list of all the commodities in trade around the world. Much less a list of how scarce or useful they are for every given possible economic use of them in every possible region and community. Should we build a new iron mine, or a new coal mine? A coal power plant, or a wind power plant? Which benefits the world community more in the long run? In principle, this is a simple optimization problem - but even the sheer amount of possibilities and alternatives is daunting. And that's before you take into account the preferences of individual humans - some people like cars. Some don't. The first group will benefit from more cars (and roads, and gas pumps) being built. The other will be harmed. And these groups don't agree on anything beyond this one thing - for every question to be decided, you have many different groups, with little overlap of the groups on a different question.
It shouldn't be surprising that one of the things the faux-communist countries did was try to limit human choice. Not only were they usually few or no alternatives to a given product ("why make three kinds of coats, when one will work as well and be easier to produce?"), there was also a push from the authorities to encourage uniformity. If every human wanted the same things, in the same proportions, the idea goes, economic calculation becomes much easier, and perhaps even possible. You may not think of things like compulsory public education as authoritarian, but in their core, they are - they're trying to build up a culture that benefits the particular arrangement of society. The same goes for media, of course.
But this is already horribly long, so let me spare you the next few pages of deeper digs into how socialism, communism and free-market capitalism work, and just give a quick summary:
No, communism doesn't have to be authoritarian. However, there needs to be something that makes defection worth less than full cooperation. There also needs to be something that allows scarce resources to be allocated. The historical solutions to these have been:
- Keep the community small. This simplifies economic calculation, and allows the altruism built into us through natural selection to work pretty well. However, individual productivity of labour drops like a stone, and if you do this to a large population, you get massive starvation etc.
- Let the whole economy be controlled by a central authority. Planning committees will decide what everyone needs, and how to make that. Any deviation must be illegal, and enforced through violence. If you want something else, you must appeal to the planning committees. Everything is controlled politically. The way the authority is chosen can be essentially arbitrary - they may be democratically voted for, chosen by a proxy, picked by some computer algorithm or based on measures of merit etc. Different people may call these different approaches authoritarian or non-authoritarian, but IMO that's a confusion; it certainly matters how the authority is chosen, but the crucial point is that individual choice is removed.
- Let people own and trade any kind of property. If I prefer beer and you prefer wine, we can trade what we have, and each end up richer. This bartering system tends to over time develop into a money system, and instead of just trading commodities, you'll quickly find people trading tools, machines, land etc. Of course, this isn't communism anymore.
- Change humans or human culture in a way that they makes them willing to cooperate even when it isn't to their (reasonably immediate) benefit.
The first is pretty stable. But it also puts limits on division of labour, and thus individual productivity. The second can only be stable with routine use of violence (or threat of violence). The third is pretty stable as well, and encourages division of labour to the extent we're familiar with, where there isn't even one person in the whole world who understands everything about making something as deceptively simple as a pencil :) The fourth may be possible to do, but would be horribly unstable - you get a runaway positive feedback loop, where every "mutant" gets advantage that makes the mutation more frequent in the population over time. To be practical, you would need some authority to enforce genetic or cultural purity, to weed out the mutant, the heretic, the unclean. A God Emperor might work :)