Joan of Arcadia/WMG

The title character is the second coming of Christ.
The show never bothered to explain why Joan was singled out to have multiple encounters with God. Even in her own family, there are people in more need of divine guidance than she is. One explanation is that, although she appears to be a normal teenage girl, she is the next incarnation of Christ. God is naturally concerned with his daughter's upbringing. Even her initials are JG, one stroke away from JC.
 * On that note, consider the first names of the actors playing her parents.
 * Wouldn't she be the third coming once you include Joan of Arc?

The main character is a psychic with powers of limited premonition, which manifests itself as visions from "God."
It explains, among other things, why the question of the existence of Satan is never fully addressed, and why an irreligious girl would begin to have visitations from God.
 * There is one storyline where Joan thought it might be Satan and not God talking to her, and another storyline where another character whom God has spoken to is introduced.

Joan's God is one or more of the future people from The 4400.
They use the same "ripple effect" method to achieve their ends.

"God" isn't a creator God of the universe, but a trickster god or Q-like being who got bored.
That's why s/he refuses to answer any questions and regularly causes the problems that Joan later has to deal with (like in the make-up episode). And it explains some of the changes in the second season. S/he is just using Joan and her friends and family for entertainment.

"God" is a Creator God who is also a Trickster.
Just because He's making the world a better place doesn't mean He isn't finding this HILARIOUS.

Grace Polk will move to Oklahoma City and become a cop
She'll eventually change her name to Grace Hanadarko and be visited by a guardian angel after accidently running someone over in a drunk driving accident