Alright, @bacchist, let me try to comprehend this. In a Communist society, the government would (ideally) redistribute goods in such a way that every man would be compensated according to his needs. Every man should then (ideally) work according to his ability, except there would be no actual economic incentive to do so.

As far as I understand it, the argument is as follows: Anarcho-Communism would eliminate the need for any incentive, since freedom of association would instead allow the rest of society to simply penalise those who are unproductive,

"[...] if you are absolutely incapable of producing anything useful, or if you refuse to do it, then live like an isolated man or like an invalid [...]"

But it still doesn't explain how to determine what is needed. What if, hypothetically speaking, everyone decided to produce the exact same thing? How would it, for instance, be determined who should carry out the most strenuous manual labour?

"Choose yourself the producing groups which you wish to join, or organize a new group, provided that it will undertake to produce necessaries."

If everyone is allowed to choose for themselves the work they want to do, then, surely, no-one would want to work harder than others. And if everyone chose the easiest job and simply produced the same thing, they would be unable to punish each other for being "unproductive".

Even though everyone would be forced to be productive (owing to freedom of association), it seems to me that – as @dwinblood pointed out – there would be a need for some sort of central planning to ensure that this productivity be put most efficiently to use.