This opens about 10 different threads which could all lead to interesting discussions. I've been investigating approaches large organizations apply to these challenges and hope to publish my recommendations soon.

Regarding Vic's original point, we feel hierarchies can be useful ways of thinking about requirements but you also need to break free of them when it comes to prioritization. The ScrumWorks theme tagging mechanism allows you to retain those hierarchies even as you reach deep inside them for the juicy (high ROI) bits you need to do right now.

We need a 1-dimensional prioritized transformation when it comes to *scheduling work*. Our ability to apply multiple tags means you can apply as many groupings as you find useful for the complexity of your problem, whether that's a Work Breakdown Structure, a traceback of split epics (megastories) and goals, or a traditional numbered requirements document.

Michael James
Software Process Consultant (who spent years doing aircraft/spacecraft real-time embedded systems thus can appreciate Nick's challenges)
http://www.danube.com