Leo at ZenHabits has taken a stab at simplifying the GTD system with Zen To Done (ZTD): The Ultimate Simple Productivity System. He says,
ZTD captures the essential spirit of the new system: that of simplicity, of a focus on doing, in the here and now, instead of on planning and on the system.
If you’ve been having trouble with GTD, as great as it is, ZTD might be just for you. It focuses on the habit changes necessary for GTD, in a more practical way, and it focuses on doing, on simplifying, and on adding a simple structure....
ZTD attempts to address five problems that many people have with GTD. I should note that GTD isn’t really flawed, and doesn’t really need modification, but everyone is different, and ZTD is a way to customize it to better fit different personality types.