What would you think are "sub-skills" of Graphic Design?

I’m reading this book: The First 20 Hours: How to Learn Anything . . . Fast! by Josh Kaufman. He suggests when you want to learn something, you should break down the skill into “sub-skills.” So, for graphic design, I think this is a decent breakdown:

Layout
Color
Typography

What am I missing? Having no formal Graphic Design training, I’m not sure where to start? I know there are some graphic designers on Fluther, so please advise… thanks!