My own analysis of the Repugnant Conclusion is that its apparent force comes from equivocating between senses of barely worth living. In order to voluntarily create a new person, what we need is a life that is worth celebrating or worth birthing, one that contains more good than ill and more happiness than sorrow - otherwise we should reject the step where we choose to birth that person. Once someone is alive, on the other hand, we're obliged to take care of them in a way that we wouldn't be obliged to create them in the first place - and they may choose not to commit suicide, even if their life contains more sorrow than happiness. If we would be saddened to hear the news that such a person existed, we shouldn't kill them, but we should not voluntarily create such a person in an otherwise happy world. So each time we voluntarily add another person to Parfit's world, we have a little celebration and say with honest joy "Whoopee!", not, "Damn, now it's too late to uncreate them."

And then the rest of the Repugnant Conclusion - that it's better to have a million lives very worth celebrating, than a billion lives slightly worth celebrating - is just "repugnant" because of standard scope insensitivity. The brain fails to multiply a billion small birth celebrations to end up with a larger total celebration of life than a million big celebrations. Alternatively, average utilitarians - I suspect I am one - may just reject the very first step, in which the average quality of life goes down.

This tends to imply the Sadistic Conclusion: that it is better to create some lives that aren't worth living than it is to create a large number of lives that are barely worth living.

Average utilitarianism also tends to choke horribly under other circumstances. Consider a population whose average welfare is negative. If you then add a bunch of people whose welfare was slightly less negative than the average, you improve average welfare, but you've still just created a bunch of people who would prefer not to have existed. That can't be good.

There are several "impossibility" theorems that show it is impossible to come up with a way to order populations that satisfies all of a group of intuitively appealing conditions.