If you're using Scrum, the backlog. How big is it after each sprint? Is it shrinking at a consistent rate? Or is stuff being pushed into the backlog because of (a) stuff that wasn't thought of to begin with ("We need another use case for an audit report that no one thought of, I'll just add it to the backlog.") or (b) not getting stuff done and pushing it into the backlog to meet the date instead of the promised features.
