I've never understood why the argument against Communism always comes down to a lack of incentive to put in more hard work. I put hard work into the things I do because it nets me social power and prestige, and it would likely be the same if we all had the same material wealth.
Social power and prestige exist just fine in very small societies, which includes the upper echelon of the highest skilled minds in a large society. In any situation where each person who receives goods from society is in direct contact with the people who produced those goods, there's some mutual respect going on.
Once it's possible to be anonymous, or when the people you deal with on a day-to-day basis don't actually know about you or your accomplishments, it breaks down. The people who bring you goods don't actually know what you do or don't do. It's impossible to create social pressure on weaker-producing elements. And if you do very well at your job, nobody will necessarily know, or care.
People who are driven by the good feeling of a job well done, and in showing it off to others, won't work any less hard under communism. But what about people who don't really care about social power? Art is one thing, but what about farming, or factory work, things where you're terribly sore at the end of a hard week? Plus, it's hard to say "Okay men, if you can produce these widgets 20% faster, you'll get a raise". "You'll win a sticker on your progress report!" just doesn't have quite the same impact.