What's my favorite thing about documenting requirements?

My favorite thing about documenting requirements is talking with someone who's an expert at a particular job, but might not have a sense of how much their work contributes to the big picture, to the bottom line. I capture what they do, and make sure I got it right. Then I put it into the context of the big picture, and hold that view up for them to see. The reaction is usually something like, "Oh, wow! So that's what this does, how that works, where I fit, what they do!" Almost always, they get a bigger sense of themselves and their importance, and a better understanding of those they interface with.