It seems a funny question for an ICC member to ask, but LoneLondoner did! Is it a trick question? We know what Marx said about this and I paraphrase: communism means, do what you can and receive what you need. That's a materialist point of view. He also said communism is "the negation of the negation" the original negation being capitalism itself. For me this is easier to grasp and respond to. So, if we decide what the most awful thing about capitalism is, then we can be sure that whatever communism may be, it won't include this most awful thing. So in some ways it's very personal.
For me the most awful, terrible thing about life under capitalism is FEAR. At the simplest level, fear of not having a job, or losing the job you've got, or having a job that stultifies. (Dont they all eventually?) This leads to fears about being able to feed the kids adequately, getting them to school and being able to live up to your responsibilities as an adult. So then there's feelings of being inadequate that constantly arise from this; fears of not being sufficiently educated, even by bourgeois standards, to properly compete with the smart guy down the street. Persistent half acknowledged fears about health- what happens if I'm ill, or incapacitated, who'll manage things? Continual worries about money, and what'll happen tomorrow; how is it possible to plan anything when you don't know what unforeseen horrors may turn up unexpectedly out of the blue? It's the total inability to properly plan for and predict the future that renders life under capitalism nothing but a ridiculous gamble; no matter what you do, no matter how hard you try, no matter how much you might want to make things better. "As flies to wanton boys are we to the gods: they kill us for their sport," as Shakespear put it. Doesn't this sum up life under capitalism? Isn't this why constant fear accompanies so many of us from the minute we wake up everyday till when we finally get to sleep and perhaps relive these fears in disturbing dreams. This all stems from the awareness and the fear that we have no control over anything at all, that life is all chance. And this misery even applies to the bourgeoisie, who also have little control over anything, but assume that this is the nature of human life.
But it isn't! Under communism all this fear disappears; because instead of living against each other in persistent competition for scarce necessities, we will live in communal solidarity - even the solitary will no longer have to live in punitive isolation - and we will work together to produce enough to abolish all kinds of scarcity. Indeed, the identification of all the scarcities we suffer at the moment in silence, and their satisfaction, becomes a major function of communist existence, as we will all want together to develop our potential. Education will become a way of life. The freedom to be our goal. The freedom from fear our task.