The body inside the clothes. Some people's frame brings a lot of structure to their outfits...they have angles, they carry themselves strongly, their bodies have clean lines, etc. Other people's bodies are just more gentle or rounded in line. An unstructured garment will look totally different on these two different people.
And then there's personality -- I don't know how to define this, but I know some people who are "crisp" in their bearing and how they speak and move. They ARE structure.
Obviously, there's not a whole lot we can do about changing what we bring to the clothes, but it's worth taking into consideration.