WFIRST will answer basic questions about dark energy: Is cosmic acceleration caused by a new energy component or by the breakdown of General Relativity on cosmological scales? If the cause is a new energy component, is its energy density constant in space and time, or has it evolved over the history of the universe?
Completing a census of Exoplanets to help answer new questions about the potential for life in the universe: How common are solar systems like our own? What kinds of planets exist in the cold, outer regions of planetary systems? What determines the habitability of Earth-like worlds? This census makes use of a technique that can find exoplanets down to a mass only a few times that of the Moon.
WFIRST will offer a Guest Observer program that supports community-based observing programs. While the baseline mission emphasizes the dark-energy and exoplanet measurements, the additional surveys carried out via the guest observer program will exploit WFIRST's unique capabilities to substantially broaden the science return of the mission. The Guest Observer program will provide broad support to many fields of astrophysics in the tradition of HST, no doubt with the same astonishing results of new, creative, field-changing science. In an extended mission, the Guest Observer program would likely become the dominant part of the WFIRST mission. HST has demonstrated clearly that the combination of a powerful facility and peer-reviewed proposals has the greatest impact in advancing the extraordinarily broad field of astrophysics research.