You have to consider the GTD "priority" discussion in the context of the other time/life-management systems which DA is setting GTD against.
I struggled for a long time with Covey's ABC/123 Daily Task Lists, and spent the last 30 minutes of each work day copying over the undone Daily Tasks (most of them) to the next day.
Generally what would happen is that I'd avoid the B (should be done as soon as possible) tasks until they became A (HAVE to be done today) tasks, and fall into the C ("low-priority") tasks because they were the most fun.
Even the most thoughtful ABC/123 pre-planning didn't help me get this stuff done. Why? Obviously there were other factors, like time, resources and energy. But under the ABC/123 system, these factors were not explicitly acknowledged. I was just left with the sense of failure at not completing the tasks that my pre-prioritizing said I should be doing.
My biggest Aha! with GTD was reading the acknowledgment that we all have HUNDRED of to-do's, and that the best plan can be shot to h*ll with one drop-by by our boss.
Now that I've transitioned from Covey to GTD, the things that used to be
"A tasks" are on my calendar, and I've weeded out the B's and C's that were really Someday/Maybe. I'm left with a full overview. And when the things that seems best to do is a silly, mindless (previously "C task") I don't feel bad that I'm doing it before getting my "B's" done.