As I understand it, a NA is something that I am committed to moving forward between now and my next weekly review, given the chance of doing so. Most errands that I could run is on my NA lists; and that's fine because I will only be looking at them when I'm in that specific part of town. I have a long list of phone calls, because they're all calls I would make, if I have the time and energy and nothing else is more important. If I'm not committed to moving on it now, even if I got the chance, then it is really not a NA but a SDMB.
In practice, however, I do stray a little from this principle, as I do (for some of my contexts, like @Office, @Home) keep things away from my NAs and on my SDMB list in order to keep the lists shorter. I do this for things that I would have liked to move on, but cannot foresee any possible situation in which that thing would be the proper thing to do. This is simply trading a small chance of not having the right thing in front of me for the clear benefit of having NA lists that are short enough to be workable.
I guess ideally, this trade off wouldn't be necessary, but after every other way of keeping the NA lists workable by breaking them up into several lists, grouping actions, and sorting actions, some actual decision making (in advance) of what will be given priority is probably unavoidable. The conclusion then seems to be that in the end committing to doing stuff inescapably is something that you need to do in advance. GTD will help you go more with the flow, but if going with the flow is all you do, then you're back to latest and loudest again.
Hope that makes sense...
Oh, and if I ever were to end up with empty (or even close to empty) NA lists, I will most certainly simply do an early weekly review and get my lists filled up again. To me that isn't even close to being a problem.