Why would the goal become 'how do we feed the most people with the resources available?' Wouldn't it be: 'how can I gain the most advantage from the resources I possess?' With or without currency, that remains the goal.
I'll admit I used to have the same view on this, where is the incentive? Why would someone do something without this immediate reward system? But it is this problem which I think would create the best (or at least most interesting) results. Without immediate reward the farmer will stop farming, why should he keep working for others? Agriculture then becomes a problem for society to solve in the most efficient way possible instead of a status quo job for a few individuals. In going back to the original point of this page, imagine the hacker community's response to that, a real, important problem to solve.
So to answer your last question, with the lack of currency/money there is no difference between them, the only form of advantage to be gained is in your social standing (interesting implications on leadership here), using your skills to help others and solve societies problems is your only way to gain any advantage over others.
I could enforce this by picking the meanest, biggest people in my town and feeding them in exchange for their protection.
You should remember that prior to the invention of capitalism and profit as a calling in the 19th century, many people in the western world lived like this.