What beings are morally significant?

When considering the morality of an action, one often considers the impact it will have on the affected beings. These beings would be considered morally significant if the effect of an action on them changes the morality of the action. Although the action is discussed most often, the moral significance of the affected being can play a deterministic role in the action’s morality. Burning a piece of paper is less morally wrong than burning a human. Moral significance seems to be less apparent, however, when considering differences between a tree, a dog, and a human.

What differentiates the beings we consider morally significant, versus those we consider okay to harm? Is it consciousness? Is it the ability to think and consider options rationally? If so, what about humans that are mentally disabled? Are humans intrinsically morally significant, regardless of all other attributes? If so, why? Can a person lose moral significance? What qualifies a being for moral significance?