1) It kills economic growth: Strong economic growth is what produces jobs, tax revenue and a better standard of living for everyone, including the poor and middle class. That's what John F. Kennedy was driving at when he said, "A rising tide (in the economy) lifts all boats." Socialism strangles economic growth in the crib by penalizing success and rewarding failure. When you loot the successful people in a society to give it to the less successful, you quite naturally reduce the number of successful people and encourage more people to fail. This leads to a never-ending cycle. The more people in need there are, the more the successful must be penalized to pay for them. The more the successful are penalized, the fewer successful people there are. This causes wealth to concentrate in fewer hands, the economy slows down, and even more people need help. It goes on and on until you get a slow economy that can't produce enough tax revenue to sustain itself. That's exactly what killed the Soviet Union, it's killing Greece right now and sadly, the United States and most of Western Europe is on exactly the same path.
2) It stifles free speech: Why is there ridiculous government propaganda in nations like North Korea? Why are most schools, papers, and colleges run by liberals in the United States? Why do liberals often try to disrupt conservative speakers on college campuses? Why are there such extreme speech codes in Canada that it practically makes some conservative arguments illegal? Why does speaking out against the government risk imprisonment in China and the old Soviet Union? Because socialism requires protection, propaganda, intimidation, and darkness to survive. Socialism can't survive honest, informed debate about its merits among people who are free to choose or reject it because it would not survive the conversation. As Reagan said, "How do you tell a communist? Well, it's someone who reads Marx and Lenin. And how do you tell an anti-Communist? It's someone who understands Marx and Lenin."
3) It leads to an increasingly tyrannical government: Freedom and socialism go together like oil and water. The more socialism you have, the less freedom you will have because socialism can't survive if people are free to choose whether they want socialism or not. People who are free to say what they want will criticize socialism's many failures. Areas that aren't tightly controlled will move towards the free exchange of ideas and goods, not socialism. So, socialism requires a massive bureaucracy that almost inevitably grows. As government grows, it inevitably becomes more centralized, more distant from the people and ultimately more menacing.
4) It creates strife and division: Socialism is all about turning people against each other. It has to be. After all, if you believe in controlling people's lives, the people who don't wish to be controlled need to be vilified. If you believe in confiscating the wealth of successful people who won't give it up willingly, then others must be convinced they're terrible human beings who deserve to be punished. "Victim" classes must be created for the socialists to defend because if everyone is responsible for himself, what need is there for the socialist? Eventually, those who depend on government for their livelihood and those that the government smears and loots to pay them off come to hate each other.
5) Socialists believe the ends justify the means: Like the pigs in Orwell's Animal Farm, socialists believe that, "All animals are equal, but some animals are more equal than others." For a socialist, the overriding concern is always promoting socialism; so process, rules and regulation mean different things for different people. Fidel Castro may have been the leader of a Communist revolution against the evil "rich people" in Cuba, but he’s worth 900 million dollars today. A law broken by a Democrat and a Republican may be treated very differently by the papers, the courts, and even the Department of Justice under Eric Holder. As Margaret Thatcher explained,
"Left-wing zealots have often been prepared to ride roughshod over due process and basic considerations of fairness when they think they can get away with it. For them the ends always seems to justify the means. That is precisely how their predecessors came to create the gulag."
One of the reasons so many socialist nations are wracked with violent protests and revolutions is because when the rule of law is abandoned, only outlaws have any hope of receiving justice.