Yeah, I've wondered this for a while without getting any closer to an understanding.
It seems that everything that some human "really wants" (and therefore could potentially be included in the CEV target definition) is either something that, if I was sufficiently well-informed about it, I would want for that human (in which case my CEV, properly unpacked by a superintelligence, includes it for them) or is something that, no matter how well informed I was, I would not want for that human (in which case it's not at all clear that I ought to endorse implementing it).
If CEV-humanity makes any sense at all (which I'm not sure it does), it seems that CEV-arbitrary-subset-of-humanity makes leads to results that are just as good by the standards of anyone whose standards are worth respecting.
My working answer is therefore that it's valuable to signal the willingness to do so (so nobody feels left out), and one effective way to signal that willingness consistently and compellingly is to precommit to actually doing it.
This is for anyone in the LessWrong community who has made at least some effort to read the sequences and follow along, but is still confused on some point, and is perhaps feeling a bit embarrassed. Here, newbies and not-so-newbies are free to ask very basic but still relevant questions with the understanding that the answers are probably somewhere in the sequences. Similarly, LessWrong tends to presume a rather high threshold for understanding science and technology. Relevant questions in those areas are welcome as well. Anyone who chooses to respond should respectfully guide the questioner to a helpful resource, and questioners should be appropriately grateful. Good faith should be presumed on both sides, unless and until it is shown to be absent. If a questioner is not sure whether a question is relevant, ask it, and also ask if it's relevant.