Unfortunately, believing that the SIAI is likely to have a significant impact on the future of humanity already implies accepting many of its core claims: that the intelligence-explosion view of the singularity is accurate, that special measures to produce a friendly AI are feasible and necessary, and that the SIAI has enough good ideas that it has a reasonable chance of not being beaten to the punch by some other project. Otherwise it's either chasing a fantasy or going after a real goal in a badly suboptimal way, and either way it's not worth spending effort on influencing.
That still leaves plenty of room for disagreement with Eliezer, theoretically, but it narrows the search space enough that I'm not sure there are many talented ethical philosophers left in it whose views diverge significantly from the SIAI party line. There aren't so many ethical philosophers in the world that a problem this specialized is going to attract very many of them.
As a corollary, I think it's a good idea to downplay the SIAI applications of this site. Human rationality is a much broader topic than the kind of formal ethics that go into the SIAI's work, and seems more likely to attract interesting and varied attention.
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.