Semantics (meaning), Syntactic (syntax), and Pragmatic (realistic considerations) are Vignelli top 3 most important aspects of design. Though there are other aspects such as discipline, appropriateness, ambiguity, and timelessness. His top three are obviously the most important to design because without semantics or a purpose to what you are doing then what is the point, you can throw a bunch of things together yet accomplish nothing. Syntax of the project is also important so that things relate and can be easily understood by your audience. While designing having a realistic design is important so that your given goal can be reached and not just a far off dream that can never be achieved.
Vignelli begins to talk about the tangible part of design towards the end the booklet. This part I found not so important in that it talks about choosing elements to help you accomplish (physically) your goal outcome, not how to mentally design and prepare to create your project. I believe brainstorming is the part people struggle with the most not how to do it once they know their end goal.