Cuba also is one of the highest developed Latin American state, ranking 2nd total and 2nd in all parameters in the 2014 UN report, apart from income.
Yes, Cuba has big deficits in democracy and freedom. But it would be very interesting to see it find other solutions than installing capitalism, which in all countries (even in those where it created overall wealth) also came with massive downsides. In China for example, sure we can look at the emergence of a new middle and upper class, but if that will ever be helpful for the vast majority of simple workers and give them better living standards than the lower part of Cubans already has is an entirely different question - besides the fact that the influx of capitalism actually did very little for political freedom in the boom states that are typically named as capitalist success stories, and definitly hasn't kicked in in China yet.
As about /r/socialism's view on Cuba, yes it is overly optimistic and ignoring the downsides. But just saying "let's implement capitalism and all will be good" is just as naive as to say that Cuba is totally fine as it is.
As about Cuba's background in communism and Marxism: Communism was always only the goal, but never the state attained by the so-called communist states. Communism is achieved when the economy consists of free associations of workers who manage their work themselves (much the way a tribal society works). That is the utopian vision that communists are still looking to realise through different social arragements and institutions, much the way democratic philosopers and revolutionaries during the feudal ages were looking for ways to implement a democratic state, without the certaincy whether that is possible in any shape.
Much of the praise Cuba recieves in /r/socialism comes from a hope that Cuba is on a good way towards implementing true communism. That somehow there is a non-capitalistic form of society that can deliberately work towards the communist utopia. This is interestingly contrary to classical Marxism, which assumes that the power of capitalism to create tremendous economic growth and personal liberty (yes, Marx saw these as advantages of capitalism compared to alternative really existing economic models!) is a necessary precondition for the path to communism.
Marxism is a tool of analysis of political economy: There are Marxists who see the USSR and Cuba as a form of bureaucratic feudalism that is called Asian Mode of Production (even though it really is a worldwide phenomenon, it was just kept up in Asia for the longest time). This mode is characterised by an aristocracy and/or bureaucracic class opposing a working class. The land and means of production are generally considered to belong to the state.