I take it as fairly obvious that in some sense we ought to be egalitarians about moral status - we (I'll leave the 'we' vague for now) ought all to be treated with the same respect and consideration and ought to be thought of as being moral equals in this sense (note that this doesn't mean, of course, that we are moral equals in the sense that one of isn't of better or worse character than another or doesn't do better or worse things). But what is the scope of this 'we' here that we ought to be egalitarians about? Most would say, inuitively, that the 'we' of which we can say "we are all of equal moral worth" is specificly the class of persons. Various theories then try to give accounts of what grounds the moral status of persons.
Unfortunately, most theories of moral personhood are themselves immoral and fail to be egalitarian enough. I take it that it would be positively immoral not to give children, infants or the severely mentally handicapped the same moral status as a normally-abled adult. Not to do so would be simply chauvinistic towards the well-developed, intelligent human being over against those who fall short. This is not a surprising thing coming from philosophers, who tend themselves to be of the very intelligent sort and tend to value such intelligence and rational thought very highly. Aristotle, for instance, seems to have thought women of lesser worth than men partly because he thought that men were more able to engage in rational thought than they were (and, indeed, held that the most valuable life was in fact the contemplative one). This, of course, is not an uncommon phenomenon - it can be seen all throughout history where human beings who do not have the particular valued qualities of the society or elites are thought of as being of less worth or consideration.
But I think as moral thinkers we ought to resist this sort of trajectory and stick up for those who cannot stick up for themselves - not only in practice but in our theories of moral status and personhood as well. Our values concerning qualities that we hold dear for ourselves ought not to blind us to the inherent value in others who unfortunately lack those qualities to the same degree as we. And this is not just for others' sakes - if we live long enough, it is more or less likely (depending on the particular quality in question) that we too will experience a diminishing of some of those qualities we value in ourselves.
Against a utilitarian conception of personhood - where the only personal moral status one has is as an experiencer of pleasure and pain - we can give the example of a person in a temporary coma who is at this time - and will for a while be - unable to experience pleasure or pain. I take it that such a person does have full moral status, despite the fact that they are in a coma and (let us suppose) experience nothing. Perhaps someone here might say that the moral status of the person here is dependent on their future status as an experiencer - that future status counts, as it were, retroactively. But if that's all there is to it, one could kill the person now, thereby preventing them from having such a future status, and one would not have violated the comatose's moral status as a person since by killing them one has prevented the comatose's having the requisite future status in the first place.
Perhaps what is meant here is that, despite the coma, the human has moral status because they are alive and existing and could experience pain or pleasure - and, indeed, do have the capacity to do so - it is only that they are temporarily blocked due to some biological damage (or drug-induced state, etc.) from actualizing this capacity. Or maybe what is meant is that, left unhindered, this human being will in fact be able to do the requisite experiencing in the future. But both of these last things goes beyond the utilitarian conception noted above - we no longer are interested purely in the actual occurent and actualizable capacities for pleasure and pain. But going beyond this conception, I think we have come up with some plausible diagnoses for why, among other things, we might be inclined to grant this person full moral status. In neither case, we must note, does this moral status depend on the ability to currently activate the qualities said to be required for moral personhood. We could say similar things about theories which make personhood depend on rationality or other mental traits. In metaphysical terms, we might say that 'being a person' is not a phase sortal.
How about grounding the personhood of those who do not share the qualities we think necessary for equal moral status in the shared interests of the community or particular segments or members of the community (a kind of contractarian approach)? I take this to be wrong too. Consider the case of a small community of adult persons out in the middle of nowhere. In fact, the community consists of only one adult person - a mother, who also has in her care an infant. Does the infant here have full moral status or not? Would the mother not caring about and then killing the infant count as murder just as much as killing an adult or be just as wrong as if it were done in a community that cared for infants? I take it that the answers to these questions ought to be yes. But what is relevant here? Well, if we take a cue from the above paragraph, we might think it has something to do with future capacities in something like the ways noted above. How about if the infant is severely retarded or if we replace the infant with a severely retarded adult? Such an individual will have no future ability to reason or do whatever the other adult does that gets them personhood on many views. Instead, it seems reasonable to ascribe their moral personhood to something like a blocked capacity for such qualities - blocked by, say, physiological defects of the brain. Again, this is similar to one of the things we said in favor of the comatose person above.
In all of these sorts of cases, personal moral status is not about having an active current capacity to do or have the qualities thought to be important for personhood. Infants, the mentally handicapped and the temporarily comatose all still have personhood. And yet their personhood is still somehow closely connected with those qualities - after all, there is strong intuitive force that personhood is somehow connected with things like experience or rationality. The common thread in these cases of those on the margins of the society of persons is that in each case we have a being which is the kind of thing that has these special qualities. They have, in teleological terms, the acquisition and activation of such qualities as a proper function of themselves. An infant, for instance, even a severely handicapped one which will never be capable of rational thought, is still the kind of thing that has rational thought as its function. In its proper environment and with all obstacles or malfunctions taken away, a person will exhibit such characteristics as are characteristic of a person. Putting it in a blatantly trivial way, to be a person is to belong to a personal kind.
We might call this a teleological conception of personhood and it seems to have important ramifications for other areas of ethics. If we accept this conception and give full moral status to the mentally handicapped and infants, we would be very hard pressed not to do the same for unborn fetuses. And if we are forced to do this, I think we would then be very hard pressed not to admit that in almost all cases aborting a fetus is morally wrong and simply a special instance of murder - a case of violence against an intrinsically valuable person of great moral status.