"Supernatural" has been one of the most consistently entertaining genre series on the air in recent years, ranking with "Lost" and "Battlestar Galactica". There's a common thread running through all these top-tier shows: emphasis on character. "Supernatural" is not about the monsters and the demons; it's about the Brothers Winchester.
The third season ended with Dean in Hell, and there was nothing Sam could do about it. The fourth season picks up four months later with Dean six feet under and back to life, more or less intact. The premiere is focused on how exactly Dean was delivered out of Hell, and why demons seem to be rather disturbed by the whole notion. In fact, they're not just disturbed, they're terrified.
Kripke throws an enormous twist into the series arc by the end of the hour. What would and could deliver Dean Winchester out of Hell? Nothing less than an Angel of the Lord, of course. It certainly explains why the demons would be terrified! Casstiel actually sounds like an angel's name (it might be, for all I know), and the effect of his presence on those without proper protection makes a great deal of sense.
There's a definite irony in the notion that Dean might be a chosen of God. There's no doubt that the divine has its place in the "Supernatural" world; the hunters rely on the articles of faith regularly. But Dean is essentially atheistic, and he sold his soul to Hell. Beyond that, his own brother was chosen to be the Antichrist Superstar! So Dean's new "mission from God" should have a profound effect on Dean's perspective.
On the other side of the coin, Sam has spent the intervening months honing his abilities under Ruby's tutelage. His ability to dispose of demons with his Jedi mind powers is pretty damn nifty, I must say. It makes perfect sense for Sam to step down that road and allow Ruby to train him as she always wanted, given what was presaged in "Mystery Spot". The fact that he's hiding that from Bobby and Dean is, of course, far more worrisome.
This has many possibilities. Just how far has Sammy walked down that dark road? Is this the payoff for the enormous amount of foreshadowing in the third season, all pointing towards Sam's ascension to the demonic throne? Was Sam trying to use his abilities to take enough control to save Dean, or at least gain revenge? (And if they couldn't keep Katie Cassidy for Ruby, why did they have to cast someone a thousand times less effective in the role?)
I can think of two possibilities. First, Casstiel is aware of something much, much worse on the horizon, and knows that Sam's unique abilities need to be harnessed for the right side. Dean is the one person who can do that, and he might even have unique abilities of his own now. For both brothers to be so important, that secret history that Mary had must have been pretty damn interesting. But two brothers, using abilities of light and darkness to win the war against the unleashed hordes of Hell? Sounds fun to me!
The second possibility is a lot darker, and follows in lockstep with the aforementioned foreshadowing. What if Sam's choice will lead to his inevitable descent, as predicted throughout the third season? What if the emerging threat is Sam himself? It would make sense for Dean to be chosen to oppose and perhaps save his brother, given the sacrifices they regularly make for each other. It would even make sense from an overall view. If the series is meant to last five seasons or so, then the fourth season could cover Dean's attempt to redeem Sam, followed by their combined assault on the demonic horde.
Whatever the case, Dean's association with Casstiel will challenge his notions of faith and his denial of God. Casstiel is, for all intent purposes, Ruby's opposite number (though clearly far more powerful). Sam's journey is already bathed in darkness and should prove entertaining as a result. With the typical "Supernatural" emphasis on character, I'm excited to