I started the thread "scheduling action items". I also reference the recent and related threads "too darn many todo's" and "comparing GTD to other systems".
I think the points in these threads identify serious flaws in the GTD system, or at least in the system as it is defined in David Allen's book (perhaps seminars and personal coaching offer additional information). In particular, while I like the principles the system is based on, I'm going to risk getting flamed here by stating that I think the system is lacking in its implementation prescriptions. Reviewing the board, I am clearly not the only one that sees a gap in the methodology and inconsistencies in guidance among the GTD staff when it comes to managing significant numbers of projects and next actions.
Quite frankly, I found the staff's posts in those threads to be vague and too high-level to be useful in any practical sense. I think all of the postings by users who have "adapted" the system and think its great because it allows you to "adapt" it are kind of humorous. Its not a very complex system; if you adapt it enough you've created your own system.
I think GTD needs another book dedicated solely to implementation of the system. I believe I saw in a thread that the next book may focus on principles. This is the wrong direction to head in. Enough about the principles and benefits and more on implementation.