I think this is a good checklist to help you understand when a stated requirement needs further investigation. I recently posted on the subject of Why Requirements Are Hard on my blog at
http://outofthetriangle.wordpress.com/2008/03/09/why-requirements-are-hard/I have the view that gathering requirements in fundamentally difficult because of the way our mind works but I suggest a few ways that you can ensure requirements more accurately reflect what the customer wants.