SarahC:

The point is, if you want universal morality, you have to exclude some points of view. If you want to be Sam Harris, you have to declare you won't listen to some people. If you want to claim that science can resolve questions of morality, it can only do that among people who already agree on a number of fundamental values. Nobody else counts; you have to override them, perhaps by force.

That is indeed so. But the additional trouble is that arguments based on "the well-being of conscious creatures" and such fuzzy concepts won't even be able to provide a clearly defined position with which people could genuinely agree. All they will provide is a rallying point for some existing ideological forces that will gather around it guided by emotion or interest.

Moreover, once "science" is taken to be authoritative for resolving moral questions, then in any realistic human society, it is only a matter of time before the very notion of "science" degenerates into a fig-leaf for ideology and venal interest. (Which has in fact already happened to a large extent in some fields that modern governments rely on as an authoritative guide for policy.) Thomas Hobbes put it best:

[The] doctrine of right and wrong is perpetually disputed, both by the pen and the sword: whereas the doctrine of lines and figures is not so, because men care not in that subject what be truth, as a thing that crosses no man's ambition, profit, or lust. For I doubt not, but if it had been a thing contrary to any man's right of dominion, or to the interest of men that have dominion, that the three angles of a triangle should be equal to two angles of a square, that doctrine should have been, if not disputed, yet by the burning of all books of geometry suppressed, as far as he whom it concerned was able.

Harris's inability write a whole book about this topic without being able to grasp this essential point is, in my view, enough to dismiss him as an altogether incompetent thinker.