From the moment this episode began, I knew it was going to be a game-changer. On "Supernatural", that means a lot of darkness and a lot of angst. That's certainly the case for Sam and Dean, though Dean certainly gets the lion's share of the self-loathing. Surprisingly, this was also full of Castiel Angst, which was a good bit of continuity. I'm not sure how some of the angel mythology in this episode is going to be received. I expect more cries of "blasphemy!" from the usual corners.
Let's start with how this episode changes the mythology. So it's not enough that Lilith and her ilk decided that it would be a wonderful idea to unleash Lucifer and wipe out the human race. (Despite the fact that demons used to be humans, so no more humans means no more demon, which means no future siblings.) Now we have a contingent of the angel population, stretching up to the highest levels of the celestial choir, deciding that following Lucifer would be a great idea. Which amounts to fallen angels and demons banding together to wipe out humanity and help Lucifer overthrow God.
The implications in terms of the narrative are staggering. Uriel was just the tip of the iceberg. His rampant anti-human commentary now makes a lot more sense, of course, but one could have interpreted that as a result of his role within the angelic hierarchy. Castiel (and Anna, for that matter) was closer to humanity for longer, so of course he would have greater sympathy for them. Now Uriel's detachment becomes, in a sense, the basis for his conversion to the dark side.
Taking out Uriel doesn't solve the overall problem, because Uriel was getting his orders from others much higher up the food chain. Which means that all the orders that Uriel passed down to Castiel, maybe from the very beginning, are in doubt. This alone makes it hard to know if Castiel's explanations for Dean are valid. And one might also wonder how long this rebellion has been brewing.
It sounds like it might have been going on for a little while. If so, this places Anna's comments about the reasons for her fall in context. It may not be that God isn't communicating anymore; it sounds more like one of the highest ranking angels has chosen to rebel, and has been blocking the communication. And we can see the effect in angels like Anna and Castiel. More with Castiel, of course, because his arc this season has been one of increasing doubt.
Whatever the case, Uriel's little plan backfired. Leaning the truth seems to have only strengthened Castiel's resolve. Even if he's not sure about his faith in the system anymore, he does know that the rebelling angels are wrong. And now he's going to set things right. (And for those thinking that this is all incredibly blasphemous, it's not hard to assume that God knows exactly what's happening, and doesn't feel the need to intervene, because it's going to work itself out according to plan. Isn't that the understanding when all horrible things happen? That it's part of the grand design?)
Another matter cast into doubt is Dean's deliverance from Hell. Was that truly God's command, or was it something the rebellious angels wanted? And if the rebelling angels wanted it, and if they insisted that Dean deal with Sam, does it follow that Sam's growing power is not just a problem for the demons? It might be that the rebellious angels and the demons alike see Sammy as the one threat to their common goal of releasing Lucifer.