Government spending, which acts as a demand side influence for good or ill, should be increased only when the majority of people are unwilling to take financial risks by investing their time and resources (money or what money represents). As long as most people in low-paying jobs are aiming to climb the corporate ladder, the economy moves forward. It is only in rare situations like the Great Depression that the government must artificially stimulate the economy. There also does need to be a safety net for those who can't find any work (the disabled). The basic principle remains in tact, though. If most people foresee possible advancement beyond what they could have under communism, capitalism makes sense. It's not ideal, but then again, not much of anything is what it should be.
As for social problems caused by communism, apart from the risks in the transition to communism, imagine America (or Britain) as a nation of voluntary communes. Even if everything had gone well up to this point, I'm reminded of the fact that 4% of people are psychopaths and they generally blend into society quite well. Since there would be no centralized government to punish them, they would steal goods, gain influence and so forth by force or by stealth. The only solution would be pure vigilante justice, but that would cause fear to spread and radical individualism would return as people attempted to protect themselves both from the callous troublemakers and the panicking vigilantes. Eventually, some centralization of power would be necessary, and anarcho-communism would become authoritarian communism, which, as both history and common sense show us, is about as far from a utopia as it gets. Ultimately, I'm convinced that human nature is so driven by a desire to hoard goods, even when such miserliness is unnecessary (not unlike dogs eating quickly even when no other dogs are present), that right-wing economics are inevitable. The only question, then, is whether the government will seek to benefit economically from its power (right authoritarianism) or allow a sort of privatized plutocracy (right libertarianism). I dislike both, but those seem to be the only realistic options. Do you remember the political compass? In my view, as economics move left, social policies become more authoritarian by necessity, but then that authority is used to enforce inequity rather than equality, meaning that authoritarian governments move right. I don't like any of this, but I see it as the only realistic outlook.

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"I am a Tory Anarchist. I should like every one to go about doing just as he pleased- short of altering any of the things to which I have grown accustomed." (Max Beerbohm)