I think the crux of the problem with socialism/communism (besides their total disregard for individual rights, as Daciple so eloquently pointed out) is their reliance on central planning. This is a direct reflection of the elites' desire to elevate themselves above the Creator of All.
Human knowledge, no matter how vast, is always inherently limited. We can never see all or know all. We may delude ourselves into believing that we do, but we can never fully understand the multitude of forces behind even a single event. It is extremely foolish to believe that one person or even a group of highly intelligent people are able to plan an economy. Central planning entails knowing what and how much of each thing each individual person needs or wants at any given time, which is always in flux. It's literally impossible to know! Yet advocates of central planning would have you believe that their knowledge is as great as the Creator's.
Furthermore, prices of goods are not arbitrary in a free market system, and they actually communicate a great deal of complex information that allows an economy to essentially plan itself! The price of a good or service communicates how much of a good or service is available, as well as how in demand that good or service is. If supply is very limited or demand is very high, the price rises, and if supply is great or demand is low, the price will fall. When a price rises, it gives entrepreneurs an incentive to produce that good or service, thereby increasing the supply. Likewise, if too many people jump on the bandwagon and too much of a good or service is produced, the price will fall, and people will begin to invest in other things that are more likely to produce a profit (ie, that other people value more). Central planners presume to know all of this information (which is of course constantly changing!) at all times and purport to know what you want and need better than you do! Central planning is doomed to always fail because it is based on arrogance.
Those who say we don't need money are clearly not understanding how difficult life would be without it! Imagine for a moment that you are a furniture maker. Think how difficult it would be to meet your basic everyday needs by trading furniture! You want some food to eat or some clothes to wear or you need a new phone. So what the heck do you do if none of the food or clothing or phone vendors need any new furniture? Do you lug all of your furniture around with you everywhere in hopes that someone will want something you have? Or do you attempt to persuade the people who have what you want to come to your location and look at your furniture, and then hopefully they will want to trade with you? Or say they do need a new table, but the table is worth far more than the food you need. What then? It's clearly a situation that is not workable in a barter model. Money is simply a convenience that allows people to easily trade goods and services, and that's a great thing!
I always say that socialism/communism (the principle of "from each according to his ability, to each according to his need") only works on the absolute smallest scale in a society, and that is the family unit. It works in a family because of the emotional bond that holds the family together. We gladly work our butts off to provide for our children, regardless of how much work they contribute, because we love them deeply and we are responsible for them. But when it comes to people you don't know, would you make the same sacrifices? How about if those people were adults capable of working themselves? The problem is that the workers in such a situation inevitably feel taken advantage of, and will therefore not work as hard or produce as much value. But when people are paid a fair price for their labors, they will gladly make all kinds of sacrifices in order to provide for the needs of others, regardless of whether or not they know or even like those other people. That's the beauty of the free market.
