Marxism isn’t Communism, though. Many people don’t realize that distinction.
Both answer and response are blatantly false. Marxism is communism, in that the term “communism” comes from Marx. The Communist parties of the world were once Marxist parties called “Social Democrats” but after the failure of the German revolution because of the reformist Social Democratic Party the SDs (notably the Bolsheviks in Russia) decided to adopt the name “Communist” instead so as not to share the name with the shamed party. They took the name directly from Marx, who begins Manifesto, “There is a specter haunting Europe - the specter of communism.” It is that specter about which he writes, therefore naturally Marxism is communism.
What trevorsnow probably meant but did not explain polemically is that so-called “Communist” parties around the world distorted the writings of Marx for their own opportunist aims and therefore basing an analysis of communism off of the actions of these parties is not an analysis of Marxism itself.
1. Your narrow definition of “human nature” is unfounded, especially because the same sort of principles as represented by communism work perfectly well in other contexts. Humans are inherently interdependent and succeed more often when working in cooperation with others, rather than in isolation (which in itself is impossible). The idea that “humans are inherently selfish” as a critique of communism falls because communism does not assume that humans have to give up their selfishness; the economic and political formulations of society would be built in such a way that to appease one person’s “selfishness” necessarily includes working towards the improvement of the group. There are other claims of “human nature” besides selfishness - hatred, discrimination - but the answer remains the same: these problems do not disappear in a communist society, they simply cannot be carried out in such a way that results in the exploitation or marginalization of other people. If economics are organized collectively, it really doesn’t matter whether or not you like the people you work with - you do your work because it helps you; it’s just an extra bonus that it helps others.
2. Inherent in capitalism is the eventual unraveling of the “checks” against capitalism. Because the capitalists (those that own the means of production) want to raise as much capital (money) as possible, they will do whatever they can to maximize that capital. Regulations and checks cost them money. Therefore, they will do all they can to avoid regulations and checks. This is obvious. But then you must look at the political system, which cannot be separated from the economic system in critiques of capitalism. Capitalist politicians that come into power through semi-democratic voting structures must get voters to believe that they have their best interests in mind. They do this through campaigns, which cost enormous amounts of money. Therefore, those who are the politicians have a large amount of money to start with and/or receive donations of large amounts of money. The only people with large enough amounts of money to give are the capitalists, therefore the politicians are either capitalists or indebted to capitalists, usually both. Once those politicians come into office, therefore, they do exactly what they want to do - avoid regulations to raise capital. This is all part of the capitalist system. You can’t use a corrupted system to reform itself, just like you wouldn’t use rusty steel wool to get rust off a piece of metal. The tool must be clean - therefore the tool is a worker’s revolution.
3. Social democracy is all well and good until you get into a crisis - like the one we see now. Less so in Scandinavia but definitely in Canada, France, Belgium, and the Netherlands, the economic crisis that we face today - with huge debts and failing currencies - has caused the former bastions of liberal democracy to start to crumble, implementing painful austerity cuts, eliminating those all-important “checks”, and harnessing politicians to the power of global capital (think IMF and European Central Bank) rather than to the will of the people. These crisis occur specifically because of capitalism itself - the banks in trying to raise capital for themselves messed up the lending to individuals, corporations, and countries around the world, throwing the economic systems that depended on those banks into chaos. It was capitalism that caused social democracy to erode, so if you love social democracy so much, the only option is to overturn capitalism.