Here is what I think, and I'd love to hear some criticism and reactions to it. I'm far from well-read in economics or philosophy.

Capitalism is the same as anarchy to me. I think that capitalism is what happens when everyone acts "rationally" selfishly.

The purpose of a society/government is to restrict capitalism. If everyone were allowed to act perfectly selfishly, we would have chaos, and life would suck, so we sign the Social Contract and we give up the right to some selfish actions in return for a guarantee that others will do the same. We are protected from each other in some respects, and in the remaining respects we are allowed to act selfishly and exploit each other just as we would otherwise.

How "capitalist" or "socialist" a society is considered by most people is dependent on how many capitalist tendencies are regulated or restricted by the government. While the existence of any government at all is therefore socialist, the intrusiveness of that government determines where it lands on the traditional spectrum from capitalism to socialism.

Since capitalism is the result of unbridled greed, it results in the unbridled consumption of all available resources toward the production of wealth as efficiently as possible. This inherently leads to exploitation of both other people and natural resources expeditiously and thoughtlessly. In addition to causing most (if not all) social problems, this also causes all environmental problems. The environmental aspect is one many people forget about.

Socialism is necessary not only for the good of the people in the short term, but also for the good of the environment and therefore the good of the people in the long term. Unless we move toward a more socialist global society, capitalism will continue to exploit people and the environment as recklessly as it currently does, and this can only lead to disaster for the human race.

The key, as my colleagues have mentioned in this thread, is figuring out a good way to scale actual socialist governments.