Mmmm.... I bought David Allen's book back in 2007 and integrate much of his suggestions into my life, and I don't recognize much of it in this page. My takeaway from "GTD" is a lot simpler than anything here - triage your to-do items and keep an organized calendar and filing cabinet. The end goal is to maximize the time you spend on things you need to spend time on and minimize indecision through deferment. For example, any single task that can be accomplished in five minutes should be done now regardless if it throws you off because the baggage associated with worrying about it counts more than actually accomplishing it. Things that will take more than five minutes should be pushed (and scheduled) to an appropriate time to do it, and then prioritized. It's actually a really simple philosophy to adopt because the primary hang-up is "learn to like filing", not "learn to like making lists." Allen isn't all that big on lists because they become another artifact; instead he favors pushing things into a schedule so that you can legitimately not deal with it at all until it comes up again (and if you keep doing this until it's no longer appropriate to deal with it, then it wasn't all that important anyway). The biggest investment a GTD acolyte is likely to make is a label-maker and a filing cabinet, and label makers are fuckin' fun.
I only started using GTD this year. He has some brilliantly simple ideas hidden in an awfully boring book. What got me into it was the great system to plan. Instead of improvising planning and remembering everything you need to do, take one moment of the week to plan for the next week(s) so you can forget about what you need to do until you need to do it. I started using Asana for school, making a project for every course I had and putting all the assignments, readings & finals in there and plan milestones for them. It's such a great feeling not having to think about what I need to do when. The peace of mind is better than the productivity gains, I think. Now I use Todoist for everything. All my todo's have a due date and it allows me to keep track and focus easily. It's weird, I am much more motivated to start when I put something in there with a due date. I plowed through 14 todo's today, most of which I could have done any time in the next week.
Something Ryan Holiday mentioned in an otherwise forgettable and glad-handing book is that every night, he writes down what he's going to accomplish the next day in normal-sized letters on a 3x5 card. He said he finds that it's just enough space for a reasonable amount of goal-setting. My schedule has become so externally-driven that there's absolutely zero point for me to plan anything more than appointments.