This is my perspective on the responsibilities of an OpenStack PTL. These responsibilities are in addition to those which may be delegated to project czars, but it's up to the PTL to ensure that they are all met.
Serve as a point of contact to the community. You will receive endless emails, IRC pings, and meeting invites from inside and outside your program. Given that time is a finite and unscalable resource, you end up being a communication coordinator more than the end-all-be-all of answers.
Foster the community. New contributors are real people with various experience levels and backgrounds who often require reassurance, direction, and mentoring to become productive community members. Making yourself available makes OpenStack much more approachable to newcomers, thus ensuring the project's long term health and a steady supply of reliable contributors from which to select core reviewers and czars.
Manage community logistics. Weekly meetings. Midcycle sprints. Design summit sessions. Design specifications. Code reviews. Conflict resolution.
Maybe write some code. But probably not.
So, in summary: community, community, community, backup to all the czars, and if there's any time left, code. Oh, and you're also expected to be an open book for deep technical knowledge on the project, the source of long term vision, and torch carrier of all tribal knowledge.