Good and Evil

[Neo-Scholastic]

What does this essential nature consist of? The best I can come up with is that it's a minimal set of properties sufficient to distinguish human beings from other things in the cosmos. Although many possible sets of sufficient properties can be advanced, I have doubts that there exists a single minimal set. Deciding what criteria makes a thing human thus becomes a judgment call.

Actually, the notion that identity comes from a particular set of properties is something that I would prefer to avoid. It leads to the inescapable paradoxes of nominalism and conceptualism. Lately I’ve been a bit cavalier when talking about nature, essence, and identity. I should clarify.
I hold to the moderate realist position. I do not think, like Plato, Forms/Ideas have independent existence. (For the sake of simplicity I will use the capitalized word ‘Form’ for the philosophical meaning of both form and idea.) Nor am I ready to dispense with the notion that reality excludes any objective unifying principles. Or to say it from the other direction, I do not believe that people differentiate universals and particulars arbitrarily from a contiguous reality. Forms are manifest in various instances that partake of the Form, to one degree or another. To intellectually conceive of a Form is not synonymous with imagining it. For example, most people can conceive the Idea of a chiliagon (a polygon with a thousand sides) but very few can fully imagine such a thing
Where I differ from Aristotle and the Neo-Scholatics, is that I do not think types of sensible bodies have their own real Forms; but rather, combine pure Forms within substances to manifest. The hylomorphic union of the Forms and substances give things unique potentials. So I say that if you want to know the essence of a thing it is a matter of knowing 1) the Forms of which it partakes 2)what it is made of and 3) its dispositional properties.
Therefore in one sense I agree with you. Properly identifying what a thing is and of what it potentially become (i.e. its essence) is a judgment call, but it is not an entirely arbitrary one.

… it [essentialism] remains a philosophical preference we would be wise not to over-rate.

People should take care when applying any philosophical principles.

Without raising an "objective moral imperative" (a term I've not heard before)…

I linked together three concepts. Objectivity means that everyone shares a common reality even if their level of understanding about it varies. Moral means the conduct necessary to maximally manifest a person’s potential. Lastly, imperative means an unavoidable obligation.

By linking them, I say people share a common reality and understand that reality by applying reason to experience; that reason applied to experience yields knowledge of the real Forms, materials, and dispositions that constitute the essences sensible bodies; that each human being partakes of a common essence; that people face an existential choice to either conform to that essence or act contrary to it; that to act contrary to one’s essence is to be less of the thing that one is (i.e. less good); and that therefore to be and remain human entails the obligation to conform to the essence of what it means to be human.