It is pretty clear that the word "structure" can apply to the overall shape and organization of something. It can also be used as a term for something built by men, or nature. The term "alignment" is a little more specific - it really refers to the geometrical organization of something, and the angles of the varying pieces. The term "form" has the connotation of something more dynamic. As a body moves, it will continually change the "alignment" of the parts. The "form" in terms of movement usually includes some idea that there are more and less ideal approximations to some pure pattern of motion.
Given this, what added value do we get from using the term "structure"? I think that the connotation may be a little different - there seems in my mind to be an implication that the object has some solidity, as well as a correct pattern of motion.
It follows that we should be looking at how the underlying "structure" is supported, in both static and dynamic senses. You may have everything aligned correctly, and you may preserve proper form as you move, but you could still be inefficient, and you could still be weak. The movement must be correct externally, and must be correct in a subtle fashion as well. In addition, the tension, the usage of the musculature must also be correct. Relaxed, efficient, yet powerful. When these conditions apply, we can probably say that someone has "good structure".
