I was under the impression the stated (by Eliezer) problem was that "X is wrong" is a simple assertion, which is almost certain to change no-one's mind.

Reconsider the comment to the hypothetical LW meeting I imagined. Do you really believe such a comment would be "downvoted to oblivion"? Yet that comment is also a simple assertion.

Of course it also has connotations of "What are you, some kind of hypocrite?", which isn't exactly helpful either.

These are the kinds of social rules that I was thinking of when I claimed such rules sometimes hinder moral progress. Users on this forum should not, I believe, refrain from calling people hypocrites if that accusation is relevant and supported by what they take to be the best evidence. Having said that, I wasn't actually accusing anyone of hypocrisy--just inconsistency.

(If one does think that the breach of social rules should be downvoted and is also the target of the critique, one is more likely to misinterpret the critique as a breach of a social rule and downvote accordingly, as you just did. This is another reason for disregarding considerations of social etiquette altogether.)