I think it ultimately comes down to personal motivation - how important is completing those actions to *me*? We tend to have commitments from multiple sources - relationships, work, personal goals, etc. - with varying levels of personal impact/importance. This sometimes leads to a mix of "should dos," "have to dos," and "want to dos."
GTD, with Allen's brilliant (IMHO) bottom-up approach leaves pruning the list a bit vague, and (again, IMHO) doesn't address directly linking our higher level goals with actions and projects. I fully believe bottom-up is the only way to successfully get on top for most people these days, but once that happens we're set for the next step of figuring out if the direction our current lists reflect is the one we *want* to be taking. In other words, it sets the stage for a deeper reflection on purpose.
I think at that point it's easier to get bold and 1) toss the "should dos," 2) minimize the "have to dos," (delegation and renegotiation, anyone?), and 3) maximize the "want to dos!" Applying the
Pareto principle starts to come into play at this point as well (Tim Ferriss's book helps, as does his inspiration - the original
Koch book).
Hope that helps.