The answer to those objections, by the way, is that an "adequately objective" metaethics is impossible
That's not a reason to prefer EY's theory to an error theory (according to which properly normative properties would have to be irreducibly normative, but no such properties actually exist).
Richard,
Until persuaded otherwise, I agree with you on this point. (These days, I take Richard Joyce to have the clearest defense of error theory, and I just subtract his confusing-to-me defense of fictionalism.) Besides, I think there are better ways of getting something like an 'objective' ethical theory (in something like a 'realist' sense) while still holding that reasons for action arise only from desires, or from relations between desires and states of affairs. In fact, that's the kind of theory I defend: desirism. Though, I'm not too interested anymore in whether desirism is to be called 'objective' or 'realist', even though I think a good case can be made for both.
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.