The Onion’s A.V. Club has a list of the 16 most pressing unanswered TV questions about shows no longer on the air. Of most interest to me:
7. Was the future ever saved? (The 4400)
Science-fiction series tend to paint themselves into corners when they don't have logical endpoints, and The 4400, which aired on USA for four seasons before being cancelled in 2007, was no exception.
Having watched the whole series as it aired weekly on Canadian’s science fiction network Space: The Imagination Station I was quite surprised when the series ended after four seasons without any closure. With the 4400 victimized by the writer’s strike and possibly the rising Canadian dollar we don’t know the fate of the world, or at least Seattle as played by Vancouver.
For most pressing questions about shows on the air, see Lost.