Suppose we're utilitarians. So we accept the following principle:
(U) an act is right just in case it would maximize happiness.
A non-naturalist utilitarian may make the substantive claim that whenever an act has the natural property of maximizing happiness, it thereby has the further (irreducibly normative) property of being right. If a naturalist collapses these, then their assertion of (U) seems less substantive. They are simply asserting the fact that whenever an act has the natural property of maximizing happiness, it has this very property. But that's trivial.
A complication: examples like (E) 'molecular kinetic energy is the same as heat' show us that metaphysically 'trivial' identity statements can nonetheless be informative or cognitively significant. This may be so if, although both sides of the identity statement refer to one and the same property, they refer to it in different ways (or by way of different reference-fixing descriptions). For then, in addition to the explicitly stated trivial fact, we may learn an implicitly stated substantive fact. Parfit explains:
This claim [E] gives us important information because the concept of heat is the concept of the property that is related in certain ways to certain other, different properties. (E) can be restated here as:
(F) having molecular kinetic energy is the property that can make an object have the different properties of being able to melt solids, turn liquids into gases, cause certain sensations, etc.
Here is the challenge for the metaethical naturalist: they must explain what substantive natural fact is implicitly conveyed by the likes of (U), to make such moral claims informative.
Parfit doesn't think this challenge can be met. But why not? I would have thought the obvious response for a naturalist is to adopt a kind of moral functionalism (a la Frank Jackson). The reference of 'ought' is fixed by its conceptual role R in our cognitive economy. Then, when the naturalist utilitarian claims that we ought to maximize happiness, they are implicitly conveying the substantive fact that the property of maximizing happiness has the (further) property of being such as to fulfill role R.
The problem is made vivid in Parfit's example of the naturalist utilitarian who horrifies the hospital Ethics Committee by endorsing the surreptitious murder of a patient for purpose of stealing their organs and saving more lives on net. Parfit imagines the naturalist responding as follows:
When I claimed that I ought to kill this patient, I was claiming only that this act would maximize happiness. I was not claiming that this act would have the different property of being what I ought to do. On my view, there is no such different property. Being an act that would maximize happiness is the same as being what we ought to do. Since I was claiming only that killing this patient would maximize happiness, no one has any reason to reject my claim.
But this does not seem to capture how naturalists tend to understand their own view. (The imagined 'distancing' of oneself from the ordinary understanding of moral commitments seems especially inapposite.) When naturalists make first-order claims about which natural features are right-making features, they are not only claiming that these features are self-identical (though that may be the explicit content or truthmaker for their claim, just as when scientists claim that heat is molecular energy). Rather, they are conveying the additional, important information that this feature is what the rest of us are ultimately referring to with our ordinary moral talk. That is: maximizing happiness (or whatever) is the natural property that is ultimately picked out by the reference-fixing descriptions R associated with the normative terms 'right', 'ought', etc.
There are other objections that could be made at this point, e.g. concerning whether the naturalist can really capture the full normative force of these claims. But I think the triviality objection fails.
Update: It turns out I was a bit quick in dismissing this argument. Parfit appeals to the premise that a claim like (U) is not merely informative, but more specifically that it is a positive substantive normative claim, which attributes a distinct normative property to acts that have the property of maximizing happiness (or whatever). It's much less clear whether the naturalist can meet this demand, since the "further information" they take us to be implicitly attributing seems to be more sociological or linguistic in nature, rather than attributing any further normative property.