When you look across all animals and try to determine what features they all share, you may think of multicellularity. But multicellularity has been independently developed in at least seventeen different groups of organisms, including plants, chromists, fungi, and slime molds. If you think more deeply about the characters that all animals possess, you are left with small things, i.e., molecules. At the cellular level, animals begin to all look alike. But, animals also resemble many other groups of organisms at that level. If you separate out all of the things that only animals possess, what are you left with?
The short answer is: an extracellular matrix (ECM) composed of collagen, proteoglycans, adhesive glycoproteins, and integrin. These four types of molecules are created inside, but exist outside, the cells of animals. These molecules fill up the spaces between cells and serve as structural elements. Most collagens are made of long proteins that wind around each other to form a triple helix, and the resulting fibers have a high tensile strength. Glycoproteins are large molecules which branch. These molecules have the property of resisting compressive forces. That is, they are kind of like springs; they can be squished and then return to their old shape. Integrin is the molecule that connects the outside of the cells to the extracellular matrix.
The mechanical properties of these molecules are extremely important; for instance, they often form a scaffolding that becomes mineralized and so forms bones, shells, or spicules of various animals. However, the extracellular matrix also plays a key role in the development of animals, from sponges to sea otters. In the early development of animals, some cells are immobile and form sheets called epithelia. Other cells are motile, i.e., they move inside the animal. Cell motility is necessary for animals to develop from single cells into working individuals composed of many many cells. Extracellular matrix not only facilitates not only the mobility of motile cells and guides their movements within the developing embryo, but also helps to control the transition of cells from one type to another. It appears that all animals share this complex system of development mediated by extracellular matrix, whereas all other multicellular organisms do not. This character is common to all animals as the result of their being descended from a common multicellular ancestor that also possessed this character.