Note that the target style is serious but still more chatty than a normal journal article.

It is unclear to me why this is dichotomized at all. Eliezer himself often (usually with the analogy of giving a lecture in a clown suit) discriminates between seriousness and solemnity. It appears that what you are looking for is something that appeals to members of academia, while still being readable to the layman.

I have not read very many journal articles, or written any at all, so I can't speak for how it appeals to academia, but I'd say that the readability goal has been very much accomplished, though as a fairly typical Less Wrong user I might be finding it more readable than others; however, if the goal is to make it readable to non-academia of Less Wrong caliber, the style is exactly right. It reads like a less self-referential Selfish Gene (noting also that I haven't read very many books, so this comparison might be worthless).