Mark:

A functional requirement is "good enough" when it captures the essential.   And the main essentials are identification of the required functions/processes and their inputs and outputs.  Also essential is identification of where those inputs and outputs come from/go to.

The authors of the BABOK 2.0 are aware of this.  The BABOK 2.0 is a functional spec on how to create a functional spec, and the document's authors organized it around input/process/output diagrams.

Now "just good-enough" requirements is a battle cry of the Agile community.  However, for them it really appears to be  largely just a sound bite, as none of the modeling techniques commonly assoicated with Agile gives serious consideration to input, outputs, and their interrelationships.  That is why none of the people you mention can not say anything concrete as to what are the specific attributes of a "good enough" requirement.    Things on your model like "Percieve what is good enough for your audience." and "Pick a use case senario and stick with it" are recommended steps to eliciting requirements, but - and this is critical - they do little to provide insight as to what the essentials of a requirement should be.

Tony