There is one part where I end up not knowing what to do with GTD. And that's when I can't figure out a way to break down my 'next action' into discrete actionable tasks. At the risk of being too abstruse, these are tasks that you 'measure like water' -- one part flows into the next too smoothly; you can't count the individual drops.

For example:

If I'm reading a book for research, my next action might be 'read chapter 3.' But I could read half of chapter 3; I could read a quarter of chapter 3; I could read all of chapter 3 and go on to 4; and the line between one chapter and another is totally arbitrary anyway. In terms of thinking of a discrete action, it might make more sense to view reading the whole book as a discrete action (albeit one that might take me several hours over the course of a week).

When I am in the midst of writing fiction, in one session I usually write until I get tired or bored, or don't know what's going to happen next. I can make discrete goals for myself - 'write the scene where she finds out her bike has been stolen' - but I generally don't follow them anyway.

Come to think of it, most of my concerns are about either (1) long actions that are hard to divide, except arbitrarily, into shorter ones; (2) things that should be habits but aren't. It's more of the same, over and over; and do you really put them on your next action list over and over and over?