Features are Job One
To me, the most important single thing about XP and Agile as a management process is the continuous, visible, delivery of features as specified by the customer.
An all too common outcome is for the team to take on ten stories and complete 95 percent of each. I encourage that team to score that iteration as a zero on the story value scale. It would have been better, in terms of delivering visible features, to have completed nine and let one go.
Now suppose that we have a bit more capacity, and that tenth story is a bit too big. Our options are: deliver nothing for that story, or deliver something. Business value is what our customer cares about. What should we do?
I suggest that we should go to the customer early and say
"We're going to get nine stories done, but not that tenth. We have a bit more capacity, and we'd like you to get the benefit. If you could see a way to split that story, or substitute a smaller one, we could give you another little tick of value."
We’re helping our customer toward making the date. To me, that’s when we’re really clicking. That’s when we’re really focused on delighting our customer.