Okay, so a few things.
Thank you Show for providing me was Badass Cas. I need to start just referring to him as BadCass.
First of all, his attitude, his anger. ZOMG, ten buckets of awesome on top of a bushel and a peck of YAY.
The fact that he beat the shit out of Dean. Just...fantastic.
The bank and forth between him and Dean - including the fanservice to Dean/Cas which was hilarious (not even hugely into slash, I just find it funny when fanservice is so blatant). Once again, fantastic.
The fact that Misha Collins is getting a lot better at fight choreography. (Because, let's face it - the first few time we saw him fight, it was like a stage fight in a high school production of West Side Story).
And finally, the way he went out - the chest carving. Holy Crap. Best. Move. Ever.
Sam was amazing in this episode. After all he's been through, after the angry self-righteousness that we've seen from him, to have him still believe in something good, to believe in Dean. And to handle it with SUCH maturity. He's the Sam we knew he's been all along, finally coming to the surface.
Dean - oh Dean. For all the snarking, I love how honest he was this episode. Because normally he is the picture of stoicism, shoving things down. He just let it all fly out of his mouth. And really, why the hell not - he figures it is the end, he's already made his decision at that point. He's given up and he says as much to Zach - just pleading for him to stop hurting his brothers. The fact that he comes out of it at the end - the wink to Sam - OH, the wink to Sam. Beautiful. Just beautiful.
And the final scene with Dean and Sam in the car - we haven't seen them that relaxed with each other since seasons 1-2 (maybe a couple of moments in season 3, when Dean taught Sam to fix the Impala). Point is, they were brothers again - not pawns in a celestial scheme, not warring factions, not focusing on betrayal - just the Winchester boys riding in the Impala.
And...really, I'm so glad that moment happened in the Impala. It just wouldn't have been the same in a motel room.
I will probably write more after I watch the ep tonight after work. I know the premise and I'm excited because it opens up a whole new world of possible mythologies for season 6. That is all I will say for now about that.