As a product owner, I want the system to perform emergent planning as I reprioritize the backlog (like PivotalTracker)

Emergent planning is the most powerful feature of PivotalTracker. If a user specifies the size of stories and the velocity of a team, the system schedules the stories into iterations based simply on the priority of the items in the backlog. This is powerful because it prevents iterations (and releases) from being overloaded by a person arbitrarily dropping endless stories into an iteration. It also provides a clear picture of when a feature can be delivered based on current prioritization. It really opens the process up to product owners.