I'm on record as not liking market socialism. To clarify a bit, market socialism is probably a barely tolerable transitional stopgap in the move away from capitalism, but we should push quickly for an alternative. Furthermore, I do not think "central planning" — even if such a concept were coherent, which it's not, as I will discuss later — is the only alternative to markets. But before I talk about alternatives, Let me digress and talk about moral philosophy.
Markets are as much a moral social construction as they are a technical economic social construction; indeed the supposed pragmatic technical benefits are a post hoc rationalization; markets really rest on a moral foundation. And the morality of markets is clear: there are people who are industrious, thrifty, clever, people who take initiative, people who conform to corporate culture, people who follow certain rules (and break others): these people are good, and society should reward them just because* rewarding is what we do for good people. Similarly, there are people who are indolent, profligate, dull, etc.: these people are bad, and society should punish them just because punishing is what we do for bad people.
*i.e. we do not reward them to encourage people to be good; if rewards do act as incentives, that's all well and good, but the moral function of reward is not as an incentive, but as something intrinsic to goodness.
As Corey Robin has often argued, there is also the moral status of power. People do not want to be good (else there would be no difference between the desirable and the good), so they must be forced to be good. Since ordinary people must be forced to virtue, there must be a ruling class of people who do the forcing. This class is comprised not necessarily of people who are themselves good (although they might be good), but rather those who are energetic and effective at forcing virtue on the general population. Indeed, although the ruling class will often judge its own members (often on arbitrary or sometimes sadistic standards), the ruling class must be above the judgment of the people. If we hold that the desirable and the good are fundamentally in conflict, then it is difficult to imagine a different social organization to promote virtue and discourage vice.
Markets entail both of these moral views. Among the people, those who are industrious, thrifty, etc., i.e. good, are economically rewarded, while those who are indolent, profligate, etc., i.e. bad, are economically punished with poverty and deprivation. Moreover, entry into the ruling class is possible for a person with the forceful and effective personality necessary to gain great wealth, and exit from the ruling class is possible for those who fail to maintain their monetary or economic wealth, especially from the depredations of their peers.
I will go so far as to argue that if we are going to hold these two ideas, that we should reward virtue and punish vice, again just because that's what we do about virtue and vice, and that virtue and desire are fundamentally in conflict, capitalism, not only the market but the private ownership of the means of production, is the best way of having a virtuous society.
Thus there are two broad classes of arguments against capitalism: first to controvert the previous paragraph and say that while we should reward virtue and punish vice, and that there must be a de facto ruling class to compel virtue, but that capitalism is not the best way to do that. As noted above, I don't buy that argument, but if you want to make it, good luck to you; I'd love to hear it.
The second class is to reject these ideas, that we should not reward virtue and punish vice or that desirability and virtue are not fundamentally in conflict, that the most virtuous society is also the most desirable, and thus people do not have to be fundamentally forced to be good.
Such a view necessarily entails that we hold ideas like "lazy people deserve as much of the social product as industrious people," and like "people are 'naturally'* good." This is a big philosophical bullet to bite, but I think we must bite it.
*Before you get your knickers in a knot, please note the scare quotes around 'naturally'.