If I were asked about 3 things that stood out to me while reading The Elements of User Experience, I'd have to say they were the breadth of scope of UX, the potential for change with better UX, and the UX mindset.
I think Garrett's illustration of the UX stack is a lot to take in, and justifiably so. There is a lot of inter-disciplinary practice in UX, with room for specialists and generalists. That's both a strength and weakness of the term: it means a lot of things, but as a result, can mean so little. Perhaps it's because UX can also be thought of as a collection of methods more than as a field (like human factors, human-computer interaction, human-centered design, etc.). The more I understand the diagram, the more I want to understand.
The second salient point is the potential for good if UX principles informed more design and engineering decisions. In chapter 1, Garrett outlines several benefits and potentially avoided disasters thanks to a good user experience (real and hypothetical). When I think of the purpose and the effect of UX design, I can't help but remember Dieter Ram's interview in the documentary Objectified and the similarities between user-centered industrial design and user-centered web design:
Lastly, the theme running through the book of right mindset of inquiry. We are asking questions about what is happening and why. We must know the problem and know the people who we're helping. To get closer to that knowledge, we continually hypothesize and then test these propositions.