It's hard enough to get the developer, analyst, customer combination to work together as an agile team if they aren't used to it. It's even harder to mature to a point where creating a backlog with properly authored stories comes naturally. (Remember that stories are supposed to be independent, negotiable, valuable, estimable/small, and testable)
What about estimation?
Estimation can be a tricky thing, but the agile community has come up with a structure called story points to simplify this and normalize the human error. I was recently involved in some good conversation in the VersionOne user community around how to sell the story point concept to others, and I found a great explanation of how poker planning contributes to this process.
Note to Ulisses Courel, I'd rather my infinity card over your coffee card any day! ; )