Before we get into all the things that make up elementary OS apps, there is a clarification that needs to be made. We need to understand what exactly design is about, but more importantly we want to smash two major myths:
Whether you realize it or not, you are constantly designing anything you build. It is an intrinsic part of creating something. Design is not just what something looks like. It's not just the colors and fonts. Design is how it works. When you decide to add a button that does a thing, that is design. You made a decision to add a button with an icon or a label and where that button went and the size and color of that button. Decisions are designs.
See also: Design Is Not Veneer by Aral Balkan
Design is testable. One design will meet a specific goal better than another design. Consider different types of bicycles. A folding bicycle has a different set of design goals than a mountain bicycle. Things like weight, size, and tire tread are important factors in helping the intended user reach their goals. Because we understand that design is about solving specific problems, we must also understand that we can objectively compare the effectiveness of two designs at solving those problems.
See also: Design is Not Subjective by Jeff Law