There are three points, marked with bullet points.
1) "Moral approval" is magical. 2) Reducing "This is good" to "I like this" misrepresents the way people actually speak. 3) Emotivism doesn't account for the use of sentences in a context -- which is the whole of actual ethical speech.
Emotivism is very different from projectivism. One is a theory of ethical language, and one is a theory of mind.
EDIT: Perhaps this wasn't so clear -- one consequence of projectivism is a theory of ethical language as well; see Psychohistorian below. My point was that it's a category error to consider them as indistinguishable, because projectivism proper has consequences in several other fields of philosophy, whereas emotivism proper is mostly about ethical language and doesn't say anything wrt how we think about things other than moral approval.
My point was that it's a category error to consider them as indistinguishable,
I didn't say I can't distinguish them, I said the particular attack on emotivism applies just as well to projectivism.
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.