This is a question that I have found interesting to bring up during the philosophical meetups I have taken a part in. I find it of interest to post it here as well, since UPB frequently is discussed here, and that it is a framework for evaluating moral theories. Hence, many people here might have a good understanding of this.
I really want to be as concise as possible, and since I don't find much add to the title of the topic, I will just give my two cents and then eagerly wait for some other thoughts.
As I see it, it can't be about "should", since shoulds can be used anytime you set up a goal. (E.g., if you want to go to the opera, you should turn left here.)
Also, to say that it is about interactions between humans, seems to be all to general. On the other hand, to say that it appertains only to interactions where violence is involved, appears a bit too restrictive as well as arbitrary.
So in the end, I can't really find any good definition of what a moral proposition really is, other than the one uttering the proposition says that it is about morality.
Anyways, I would be delighted to hear your thoughts about it! 
Yo sé un himno gigante y extraño
que anuncia en la noche del alma una aurora...