Rereading this after Map of the World, I was really struck by this:

The thing frowns, trying to work the unfamiliar muscles into something like anger. "You are nothing but human. You could not possibly comprehend--"

"I am not." Castiel tilts his head. "I am the will of God manifest. They are not yours. You have no right to them. And you will let them go."

and this:

"You wished for a kingdom; you will have it. All of this emptiness is your domain, and here you will remain, sealed within the nothingness you desired until the end of time. This is the judgment placed upon you. There is no reprieve."

"You don't have the authority."

"I am a messenger of God. I am the manifestation of His will. All that affects humanity is my domain." Castiel looks up; for a second, Dean thinks he can see the faintest outline of Castiel's true form, like a memory of looking at the sun.

I think this is the closest anyone's come to capturing on paper how I think of angels in my head. This feels true, somehow.