I like it - not sure why "priority' is not used more - it is such a useful construct. Instead of re-purposing it though, maybe add "tags' and still keep it super lean
What I mean is that you can separate out very different tasks by giving them far apart priorities and order and group sub-tasks/related-tasks by giving them priorities closer by. Let me know what you think.
Or if you have a way in mind about tags, would love to hear your thoughts.