Hi Mike,
I think you are getting to the heart of it, but the analogy I like is making a movie, the final result is very open-ended and coordinating the activity of getting a result very complex, also, whether the end result is a success is very much dependant on intimate knowledge of the humans who will view (use) the outcome.
I think we have to abandon the idea of a design and an implementation phase, its all basically design, I'd even go so far as to abandon UML as a design tool, to the extent that its use is premised on this separation. Building and testing prototypes seems to me to be the means to 'explore' the space of possible solutions.