Let me try my own quick summary, since I'm trying to master GTD and explain it to people at the same time.
1) A good system is practice + tools: No single software/planner, etc. will help if you don't do things like the two-minute rule, touch-it-once linear inbox processing, weekly/monthly review, etc.
2) Develop your system and the USE it: If you trust it you'll use it and it will work. I'm finally at the point where I can put something in "Someday Maybe" and trust that I can find it later. It gets it out of my head so I can get work done.
3) Mind like water and flow v. discipline: GTD requires both. Anything worth doing is worth doing well. So while there are a few things you HAVE TO do/follow to make it work well, HOW you do those things can be designed just for you. No one else has to understand it or use it. The things I think are have-tos are: Hard edges (e.g., calendar v. next actions, etc), clear next actions (next observable behavior), regularly checking NA lists when in or planning to be in the context, and regular reviews.
4) Building and practicing your system so you can trust it: Can you imagine if we could remember what it was like to learn to walk? Maybe you can remember learning a new skill or specialty. At first you learn the rules, create cheat-sheets, go through trial and error, ask questions, etc (well maybe toddlers don't ask questions about walking but you get the point ). At some point it becomes second nature. I think the same happens with GTD. But if you don't practice and stick with it, and find your own pace and style, you won't be able to trust your system, and if you can't trust it, it won't be meaningful (and thus won't work).
One reason GTD is working for me is that it's strict but flexible. I like its psychological orientation, too (contexts as reminders, etc.).