Here is a hypothetical. Let's say you are a Starbucks Barista. And you discovered GTD, read the book and would like to put its principles to work in your job. How do you go about it?
Well, I know nothing about the job demands for a Starbucks Barista other than what I see from the other side of the counter. I'm sure there is a list of routine tasks that needs to be accomplished. Your actual work is coming in as drinks that need to be made, and they are all deadline tasks because someone is waiting for it. You may be able to batch process some tasks, I don't know.
I'm aware GTD is really meant for knowledge workers, but I thought this was worth exploring. It is in the implementation, the adaption to your own job's needs that this system makes such a difference.