What happened to Castiel at the beginning of Rapture, and why will it impact Dean?
Heaven needs Dean to agree to something. Let's call this something 'X'.
What do I believe happened to Castiel? Doubting, uncertain of whom to trust given Uriel's betrayal, Castiel has become alienated from Heaven. Dean is antagonistic, difficult, and likeable. Dean has become someone Castiel can trust. Then Castiel learns that Heaven wants Dean to agree to X and is appalled. Castiel wouldn't be certain that X is in Dean's best interest, and if he doubts the allegiance of the order-relayer, he may think X isn't even what God wants. At the very least he would want to warn Dean, give Dean some sort of a heads up. Heaven hauls Castiel back upstairs, but instead of being brainwashed or thumped several times or scared straight, Castiel is brought before God.
If I compare Castiel to a shovel, I may find that shovel to be dull. So I take a hammer and file and make the shovel's edges sharp again. I have remade the shovel, and in the same way God has remade Castiel.
God's face is said to be terrible. What if just seeing God meant Castiel was filled with that terrible-ness? God didn't need to take a hammer and a file to Castiel, God just looked at Castiel.
Or, Heaven hauled Castiel upstairs and replaced him with a pinch hitter. I mean, how would Dean know if Castiel wasn't Castiel?
So how does this impact Dean?
After Castiel put things on the line with the Archangels over Chuck and Sam, Dean trusts Castiel. Castiel and Dean's relationship has become one of buddies, a buddyhood forged by difficult times. If Dean believes Castiel's new attitude is a willing one and not one imposed by Heaven, then Dean may follow Castiel's lead and agree to X. What do I mean?
I could see a scene where Dean, angry, confronts Castiel over the angel's new attitude. Dean asks what happened when Castiel was hauled upstairs. What did they do? Castiel turns to Dean, looks him dead in the eye and says, "I saw God."
Dean's taken aback - he questions this. He needs to believe this wasn't a punishment, but that Castiel has had his doubts and questions answered. It's a confirmation that God is aware of what is going on, that God has a plan, that God is worth following.
Dean is a human, and because humans were designed to have free will, Dean can't be ordered to X, Dean has to agree to it. With Castiel's reassurance Dean could trust the requests of the angels were righteous. It would give Dean a place to stand, and a place to stand is what Dean needs. Dean would then agree to X.