Nathan, I still don't see how you can derive morality from that. I agree that you can say that close to all people prefer to be healthy over being unhealthy. But from that, you can't say that it is moral for any individual to be healthy, because those people might prefer the bundle of eating unhealthy foods, being lazy and being unhealthy, over the bundle of eating healthy foods, exercising and being healthy. The only thing that can be objectively said is that if we hold all other things constant, it would be welfare harming to do something that would have no other effect than to make someone unhealthy.
But then the principle seems to be that we shouldn't do anything that harms someone else's welfare. Why is it significant that other people share the same preference? I mean, we can unequivocally say that people prefer to not have their preferences go unfulfilled. But I don't see why it is immoral to violate someone else's preferences if and only if the object of that preference is preferred by all humans. To each person, there would not seem to be any difference between preferences held only by them and preferences shared by everyone else.
I could see there being an argument that it's immoral to intentionally do something which results only in the lessening of another person's overall wellbeing, and because most humans have similar preferences with regard to certain things, there can be rules which tell us what we can reasonably expect to harm someone else's wellbeing without actually having to ask first. But the rules themselves don't strike me as objects of morality; rather the objective of minimizing harm to others is what is moral.
My problem really lies in saying that people's preferences in relation to nutrition are somehow holier than their preferences in relation to music because they are shared by all humans. It might be a better rule to not murder someone than to not expose someone to rap music, but that's only because it's difficult to ascertain whether or not they like rap music, and it's easy to assume that they prefer to live. Once they express a dislike for rap music, it would appear that it would be immoral to force them to listen to it, regardless of whether or not other people share the same preference,
So if Stefan's principle is saying that people universally prefer to be happy, and thus it is immoral to make people unhappy, then I can agree. But deriving specific moral principles beyond that is not a clear process to me.