I'm begining to think that the most adjustable part of GTD is the weekly review. All the structure of the lists, inbox, etc are essential and some of the weekly review must be done (for example make sure each project has at least one Next Action or if it's on hold then know what it's on hold for) but a lot of the weekly review up to you.
Like for some people it might be beneficial to look at each project in detail, see how far behind plan it is, review all the possible future Next Actions. Other people might prefer to do a quick skim of all the projects and make sure there is at least one Next Action. Some might need a "weekly" review every few days and some might get away with fortnightly.
Most importantly I think the weekly review is where the real long term planning happens, the natural planning DA talks about. And that kind of planning may be totally different depending on who you are and what you're doing.
The short term natural planning is what you do when you've got a two minute job to do.