Unless "theory which would maximize human fun" or "theory that would be well-received among people popularly understood to be virtuous" are desiderata of mine, why the in the world should I use them? It would be circular to use them to craft my ethical theory because my ethical theory says to, incomprehensible to use them because somebody else's says to, and unmotivated to use them for any other reason.
Yes, obviously. The question was in the first paragraph, not the second, which you seem to have gotten hung up on. The question, again, was: what's the justification for taking your intuitive desiderata as the most (sole?) important factor in deciding on an ethical theory?
I gave examples of some strategies for choosing an ethical theory that some other people might choose only to show that it's not obviously clear that your strategy is the sole or best strategy. So the question, again, is why do you think that particular strategy is the best one (assuming you considered others and you do believe that's best)?
In You Provably Can't Trust Yourself, Eliezer tried to figured out why his audience didn't understand his meta-ethics sequence even after they had followed him through philosophy of language and quantum physics. Meta-ethics is my specialty, and I can't figure out what Eliezer's meta-ethical position is. And at least at this point, professionals like Robin Hanson and Toby Ord couldn't figure it out, either.
Part of the problem is that because Eliezer has gotten little value from professional philosophy, he writes about morality in a highly idiosyncratic way, using terms that would require reading hundreds of posts to understand. I might understand Eliezer's meta-ethics better if he would just cough up his positions on standard meta-ethical debates like cognitivism, motivation, the sources of normativity, moral epistemology, and so on. Nick Beckstead recently told me he thinks Eliezer's meta-ethical views are similar to those of Michael Smith, but I'm not seeing it.
If you think you can help me (and others) understand Eliezer's meta-ethical theory, please leave a comment!
Update: This comment by Richard Chappell made sense of Eliezer's meta-ethics for me.