I've just come from a 90 minute technical presentation from a vendor. It occurred to me during my normal, incessant questioning of the presenter that what I really wanted to hear is what the product is NOT. That is much more helpful than the typical "scalable, enterprise-ready, application integration" blah blah drivel. 

Only in contrast to similar products can we really tell what something is.

Of course, as a sales person, I want to think that my product can do/be anything. <g>

This is semi-related to the idea of the domain diagram early in a system's life: tell me what it is not.

Also to the very important "Scope" section of an engagement definition -- what are we NOT agreeing to do. 

And something I remember from a brush with Developmental Psychology -- the importance of a child discovering the boundaries to herself -- "My hand, that is part of me, but the cat is not."

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Sometimes negative approach helps to make decision. Reducing scope is good when there is lack of time. Though I prefer choosing best, not avoiding the worst.