Bernie is not a true socialist, nor are those truly socialist countries. Socialism is a component of pure Marxist communism. It constitutes the transitional government and economy that comes before the true utopic dream of communism where such a government folds after stability is reached, dissolving into a sort of secure anarchy where there are no class divisions and workers share labor and egalitarian governance.
It should be noted that Marxism is more of a dream or a perfect model. It should not be sought as an ideology. It cannot exist in the world due to inherent greed and thirst for power. In historically "communist" countries, revolutionaries have used the ideas of true Marxism to seduce public opinion and military power in their favor. They use it to install the transitional socialist government, which never seems to dissovle. The populace, being uneducated about communism, is often none the wiser, leaving the "revolutionaries" to become the dictators of a socialist autocracy.
With this understanding, it is clear that no real Marxist communist government has ever existed. Marxism's inherent flaw is the giant power vacuum it leaves for whoever leads the movement. That person is easily elected (if they don't just take power) to the position of running the "temporary" socialist government, and it is up to them to dissolve that government to achieve the next step in the transition to communism.
Lenin, Mao, Castro never did. They simply used the ideology to brainwash serfs, farmers, and the downtrodden and poor. Communism is sexy because it depends on the most oppressed and thus the most UNEDUCATED in societies. Leaders like Lenin saw this component and went giddy. Brainwashing 101.
Anyway, what does this mean for Bernie sanders? Bernie sanders is not a "true" socialist. When we talk about socialism in American politcs, we don't talk about actual socialism as per Marxism or socialist dictatorships that pretend to be communist. We are talking about socialistic ideals within the context of a capitalistic market and democratic government. It has context.
Bernie is not proposing that we let the government control the entire economy and allocate resources and labor to the populace. He is proposing that we implement the ideas of DEMOCRATIC SOCIALISTS like FDR, Truman, Eisenhower, and of Scandinavian and European countries like Germany, Denmark, Sweden, and Norway. These people and governments are socialist within the confines of the American economic and political systems. They don't propose restructuring the government in a purely socialist way, a la Communist Manifesto.
What Bernie and democratic socialists seek is the expansion of entitlements and to objectively address social issues in America like police brutality, racial issues, the private prison industry, tying the minimum wage to inflation so that it cannot be lowered (by devaluing currency through printing money) artificailly, etc.
Economically, they are for things like free healthcare and education as they are seen as inherent rights. They also seek to regulate the economy, not as to punish successful businesses, but to protect workers, competition, and allow small businesses to have a chance. The current deregulation of the economy attempts to defeat the latter.
The case for Bernie itself is simple. He's been on the right side of every issue he has addressed for 5 decades. He has fought for the rights and wellbeing of ordinary people that are oprressed. He does not accept ANY corporate donations and is funded with tens of millions of dollars from individual people. He thinks that large corporations have far too much power, and a tiny percentage of the population is marginalizing the rest of the country, if not the world at large.
My favorite argument is taxes. You pay taxes. Do you like paying taxes? What do you pay them for, if just protection? Paying taxes is a social contract, and as a citizen, it is your right to get the government you deserve. Does everyone not deserve to be alive? To be offered the same base opportunities in life? To not have the anxiety of survival in 2015? Shouldn't paying the government cover your healthcare? Why should you be left for dead or left in debt if you are ill? Do those people deserve that? Why should your employer be able to buy a large set of policies and offer an obviously subsidized insurance plan to you (of their choosing) while pretending the value of the insurance is a portion of your wage? Anyone that is offered insurance is essentially having their wages garnished by the company whether they need it or not. Even if you do need it, they pay less for the policy than the value. So they get to pay you less than you should recieve. It's a scam. Not a benefit in the slightest.
What about regulations? Aren't they bad? This one is a huge myth. Regulations can be both good and bad. Republicans pass tons of regulations today, and they all protect private interests from competition. ISP monopolies and allocating public works jobs to expensive private companies are a great example of this. FDR passed tons of groundbreaking regulations that saved us from economic doom in the 30s. Bernie was referencing this in his speech on democratic socialism last week (you may just show them that!). Without regulations, we would have no 40 hour work week, no minimum wage, no child labor laws, no unions, no workers compensation, lax safety liability laws, etc. He took us out of the industrial revolution and into the modern world.
In the 80s up to 2000, Reagan, Bush, and Clinton deregulated the economy to the best of their abilities. Things like repealing glass steagall led to the complete loss of economic accountability in the banking sector and you know the rest.