more morally significant the more is it similar to human
I'd expand this to "the more I empathize with it". Often, I feel more strongly about the suffering of some felines than some humans.
Of course, that's just a description, not a recommendation. The question of "what entities should one empathize with" remains difficult. Most answers which are self-consistent and match observed behaviors are pretty divergent from the signaling (including self-signaling) that you'd like to give out.
Of course it's a description. I understood the original post as asking for description as much as recommendation.
The question "what entities should one empathize with" is as difficult as many similar questions about morality, since it's not absolutely clear what "should" means here. If your values form a system which can derive the answer, do it; but one can hardly expect wide consensus. My recommendation is: you don't need the answer, instead use your own intuition. I think the chances that our intuitions overlap significantly are higher than chances of discovering an answer satisfactory for all.
For a long time, I wanted to ask something. I was just thinking about it again when I saw that Alicorn has a post on a similar topic. So I decided to go ahead.
The question is: what is the difference between morally neutral stimulus responces and agony? What features must an animal, machine, program, alien, human fetus, molecule, or anime character have before you will say that if their utility meter is low, it needs to be raised. For example, if you wanted to know if lobsters suffer when they're cooked alive, what exactly are you asking?
On reflection, I'm actually asking two questions: what is a morally significant agent (MSA; is there an established term for this?) whose goals you would want to further; and having determined that, under what conditions would you consider it to be suffering, so that you would?
I think that an MSA would not be defined by one feature. So try to list several features, possibly assigning relative weights to each.
IIRC, I read a study that tried to determine if fish suffer by injecting them with toxins and observing whether their reactions are planned or entirely instinctive. (They found that there's a bit of planning among bony fish, but none among the cartilaginous.) I don't know why they had to actually hurt the fish, especially in a way that didn't leave much room for planning, if all they wanted to know was if the fish can plan. But that was their definition. You might also name introspection, remembering the pain after it's over...
This is the ultimate subjective question, so the only wrong answer is one that is never given. Speak, or be wrong. I will downvote any post you don't make.
BTW, I think the most important defining feature of an MSA is ability to kick people's asses. Very humanizing.