ODIN: Thor Odinson...you have betrayed the express command of your king. Through your arrogance and stupidity, you've opened these peaceful realms and innocent lives to the horror and desolation of war! You are unworthy of these realms, you're unworthy of your title, you're unworthy... of the loved ones you have betrayed! I now take from you your power! In the name of my father and his father before, I, Odin Allfather, cast you out!
If you're going to exile someone, this is the way to do it: thundering shouts followed by a blaze of special effects. And Thor still doesn't seem to get it.
LOKI: You know, it all makes sense now, why you favored Thor all these years, because no matter how much you claim to love me, you could never have a frost giant sitting on the throne of Asgard!
In some ways, Loki has always been exiled. He is a frost giant, after all, and he never quite fits in in Asgard. Exile doesn't need to be physical: it can be spiritual too.
THOR: Can I come home?
LOKI: The truce with Jotunheim is conditional upon your exile.
THOR: Yes, but couldn't we find a way?
LOKI: Your mother has forbidden your return.
Okay, so Loki's being a lying liar from Liarsville here. But as self-serving as they are, his lies actually serve some good. He's forcing Thor to accept his exile, and in so doing, allow Thor to take responsibility for what he's done. That's the first step to becoming a better man…something Thor needs to do if he ever wants to come home.
LOKI: This is good-bye brother. I'm so sorry.
THOR: No, I am sorry. Thank you for coming here.
Thor accepts his exile forever: meaning that he understands that he did this to himself and accepts the consequences. Way to be a grown-up, Thor!
LOKI: What hope is there for Thor?
FRIGGA: There's always a purpose to everything your father does.
That's the sliver of a way out for Thor, meaning that if he figures out why Odin did this, he might be able to end his exile.
THOR: You know I can't go home. My father is dead because of me, and I must remain in exile.
SIF: Thor, your father still lives.
Even though the whole "Odin's dead" thing is a lie, it's important for Thor to accept responsibility for his exile. I did it, I have no excuse, and I'm prepared to stay in this little Podunk world in order to pay the check.
THOR: You must return to Asgard. You have to stop Loki.
Thor's acceptance of his exile extends even to the fact that Asgard is in danger. That's dedication.
THOR: You can't kill an entire race!
LOKI: Why not? And what is this newfound love for the frost giants? You, could have killed them all with your bare hands!
THOR: I've changed.
This is important, because it means that Thor's exile serves a purpose beyond punishment. It's a learning tool…which Odin probably intended all along.
LOKI: I could have done it, Father! I could have done it! For you! For all of us!
ODIN: No, Loki.
In many ways, this is Loki stepping into exile as well: abandoning his home for the void of space. He's doing so voluntarily…though he doesn't intend to stay in exile for long.
THOR: So Earth is lost to us.
HEIMDALL: No. There is always hope.
Just a quiet reminder that exile need not be forever…not as long as there are sequels to be made.