What is GTD?

So many people talk about David Allen’s GTD System (book available here) these days, with so many different twists on how to implement it, that it can be hard to keep up with what GTD really is.

If you’d like a short reminder definition of it, David Allen has posted one on his official GTD site.

As you can see from David’s definition, GTD is conceptually very simple. While I personally depart from some “GTD standards,” I have found that it is actually easy to implement. I suspect that the trouble that so many people have with it is that they try to make it harder than it is. What are your thoughts on this?