I feel one of the best things anyone can do for themselves is have a regular review schedule (at least once a week or more).
I might need someone to help cite, but I thought I had read somewhere that the review is the one piece that people have trouble with. Although the rest of “system” is important (especially the “do” part), it is my opinion that if you really want to commit to GTD methodology and (more importantly) you life goals. You have to review where you are and where you are headed.
Breakdown: Collection: What good is collecting if you don’t look at it after you’ve collected it? Process: Are you sure this the right thing to do with this stuff? Organize: Did you place that tasks or reference in the best place? Is it time that you did something about that one project you’ve put off? Do: What are you doing? Is it really what you want to do?
I understand that you can get stuck reviewing things forever and that the end result of all of our actions should be to actually do something, but (to say my point again) reviewing is one of the most important things to change my life.