This book remains one of the seminal texts for design, and for good reason too. Tailored for a technical audience, Norman weaves the importance of psychology, behavioral economics, and associated liberal arts in creating and delivering products people love using. In doing so, we learn to appreciate embracing our human nature rather than trying to transcend it.
I can distill three takeaways from this book (from the many lessons Norman communicates):
There should be one right way to do your thing: Norman hammers home the importance of communicating to the user a cohesive conceptual model through the product's design. Allowing the user to do the same task through multiple pathways dilutes this conceptual model and hence confuses the user, decreasing the utility of the added functionality. Another way to say this is the conceptual model of a product should be a tree, with one path to any particular action.
Weigh the opportunity cost of action vs. inaction: Einstein famously said to make things as simple as possible, but no simpler. Norman echoes the same, warning both of creeping featurism and worshipping false images (e.g. minimalism) as impediments to great user experience. Creeping featurism may be the more prominent issue for corporate products, as the hunger for profit demands shipping more features to more customers.
Support the control layer and automate the execution layer: Humans enjoy controlling things, and bad at execution. Successful products complement human's strengths, and don't substitute for them. The difficulty of control and choice is that they can be wrong sometimes, so products should forgive users or constrain them to a certain range/set of actions.
I've found what makes a seminal text “seminal” establish a number of first principles, and then make a number of predictions about the future based on those first principles. While Norman makes some prediction errors (algorithmic search, torrenting, etc.), probably due to lack of technical background, he makes rather stunning predictions about the future. He effectively lays out the feature list for Google calendar and predicts Buzzfeed and the rise of lazy content through technological content. It brings home the point that human nature, and hence some aspects of product design, remain the same through the ages.