To me the biggest insight was the creation of the "Someday/Maybe" folder. In fact, I actually picked up on this during my 2nd reading of the book, as David Allen recommends doing.
In this Age of Instant Everything, it seems that everything has to be done now. Whether it's externally from customers who don't like waiting or internally when we're impatient with ourselves, I've noticed it's so easy to demand more and more speed from everybody. It makes it seem like if we don't act now like all the first movers, we'll lose the multimillion business deal or other chance of a lifetime. In a 24/7 economy, I've noticed some things can wait 7 days and it's not the end of the world. The "Someday/Maybe" folder is great for those.
My next most valued insight was the runway/10,000/20,000/30,000 ft. model. I couple that with the numerous mentions at the beginning of the book of the phrase "day-to-day." It is those day-to-day things that need attention. When I used other project management systems, they all sounded great at first, yet they didn't deal with life's realities (especially interruptions!) And when I tried implementing those at some workplaces, upper management told me they as a high-speed high-tech company could not waste any time on planning.
I've found that with GTD and those different levels, it is possible to plan and do activities that actually move you along both the day-to-day and overall life planes. I no longer see day-to-day activities as firefighting that plagues many offices today. Instead, they're energizing!