Personally,
I see two major flaws with socialism, communism, or it's kissing cousin, American liberalism.
1.) As soon as the government has sufficient authority to enforce socialist edicts, the rich and well connected will wield those levers of authority to further enrich themselves.
2.) The more 'social welfare' the State provides, the more people will 'live off the dole' and the fewer folks the State will have actually working to produce the wealth the State needs to survive.
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Why don’t socialism or communism work?
http://utahcitizennetwork.org/be-principled/why-dont-socialism-and-communism-work/
Socialism Overview
Socialism does not work because it does not reflect human nature. This unnatural state of existence, by the way, is what makes socialism an ideology – it does not reflect what’s real; it only reflects what’s imagined.
Volumes have been written about what can be described in one sentence: The imagined benefits of socialism always require government coercion to force changes in human behavior. That effort has never worked in the history of mankind. If socialism’s benefits were real, little government coercion would be necessary to achieve them. Ordered liberty, or freedom, requires some government coercion. The difference between freedom and socialism is that government coercion in a free society conforms to human nature. The same is not true with socialism.
. . .
The DNA that underlies the socialist and communist views of society is itself composed of a certain view of human nature. This is the idea that progress and social evolution are the goal of any society and that to achieve those goals, individuals cannot be sovereign institutions of cooperation, but rather mechanical parts in the engine of social progress. And in order for these parts to understand how to cooperate, a certain director must guide them – a director whose vision and understanding of the social good is greater than any one of the individual parts. The individual must understand its place in the machine, and do all it can to support the other parts of the machine to which it is attached.
How similar does this all-wise director sound to the king by divine right? Of course the director of this social progress cannot believe in any divinity whatsoever. Because to believe in a being outside of, and greater than, humanity is to understand the sovereignty of the individual.
Whether socialism or communism “works” or not depends on the ends sought by the director (or dictator). History informs, us however, that whatever the ends of the dictator, his regime is bound to fall because of two problems:
1. He establishes himself as ruler de facto; he doesn’t require governance, unlike his subordinates. However, human nature is imperfect, and all humans make mistakes. By not allowing for a check on his decisions he is more likely to eventually make costly decisions and mistakes.
2. As mentioned with liberalism, it is impossible for one person (or group of people) to know the needs of every individual at every moment of any given day. What Friedrich Hayek calls the “knowledge problem” greatly affects socialism and communism because both systems pretend to know the needs of the citizens and falsely promise to meet those needs through government programs.
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(Entire article is at the link. I found it worth reading. Zim.)

Mad Poet Strikes Again.