I'll begin with the things I liked.
I loved Castiel riding with the brothers, even though he could have ridden with Bobby. Of course I loved him sleeping (snoring, oh my god) and Dean thinking it was adorable and Sam not really knowing what to think (I think that, even after Sam and Castiel reached a point of friendship, Sam was still constantly befuddled by Castiel and Dean's odd relationship, and I love that).
I loved Castiel not knowing how to humor Sam. Oh god, so much love. I also loved that he looked to Dean, maybe for guidance, and Dean just gave him this look -- you're on your own, dude. Cas, we both love Dean, but he's not the one you look to for social cues.
Oh, Bobby.
Dean making the distinction between letting Sam do this thing, and Sam making his own decision to do this thing was amazing. Dean, you have grown.
It hurt when Dean began to experience panic and definite second-thoughts when Lucifer made it clear he was on to them. "Sam, no." God. And his little FACE when it didn't work. Beautifully and subtly acted.
It was a beautiful, if heart-breaking, moment watching Dean have to be the one to carry on hope for not only himself, but Castiel and Bobby. It was just that tiny bit of what I'd originally wished so hard for back in season four before we knew what Dean's role in stopping the apocalypse was -- Dean, the John Connor figure.
Michael, I'd think you would realize, having had ample opportunity to observe from on high, that being a snot to Dean... doesn't really work. Ha.
ASSBUTT. ASSBUTT.
The Impala tying it all together like the third main character she always was. Hell. Yes.
Castiel's return was a) fucking awesome, and b) so perfectly acted. I loved loved loved Dean's small voice at "Cas? You're alive?" and Castiel's gentle, sad look for him.
Also: "Are you God?" HEEEE.
However.
Chuck's narration got reeeeally heavy-handed at times, especially with the "life has plot holes, deal with it" (HI, KRIPKE, I CAN SEEEE YOU).
The ending was... I don't buy it. I'm not objecting to Dean trying to settle down and have a family, because it's been established over the seasons that he's always pined for it, even when he wouldn't admit to it. But. Castiel's just gone, after everything he and Dean went through over two years -- the heaviest shit of Dean's life, and events that changed Castiel's entire existence, and neither of them are the least bit upset about it aside from Cas sucking at goodbyes? And precisely why in the hell would Dean and Bobby part ways and not see each other for a long time? Their relationship may have begun with hunting and the shit they landed in, but it wasn't the basis of it. They're father and son, they both just lost Sam, I won't ever buy that they hugged and went their separate ways. Fuuuuck that.
Honestly, if the lesson learned at the end was that Sam and Dean always had a home -- that it was unconventional and neither of them necessarily understood it, but they made it work and it saved the fucking world, could there not have been a little realization to go along with that, that Dean could have a family and not carry the world on his shoulders, but that family didn't have to be a woman and child and a house in the suburbs? I'd give my left nut for it to have been Dean and Castiel and Bobby making a home for themselves, being family, sticking together and being there for each other because they all lost something.
Dean learned over the course of the show how to have a family that wasn't just Sam and John. He let Bobby be a father. He let Castiel be... Cas. He let Ellen be a mother and Jo a sister. It would have been extremely fulfilling for him to settle down with that, as much (if not more) so than starting blank slate with something he'd never had, turning his back on all the shit that was meaningful to him.
Dean and Sam gave Bobby a family too -- people to care about, and to care about him, not just rattling around in his big empty shell of a house with nothing but broken down cars and lore. Why fucking bother giving him his legs back if you're going to take away what was his reason for living?
Castiel's entire character arc was discovering the good parts of humanity, of learning how to be human-like himself, and cope with an earthly existence. Why the hell would you throw all that away to send him back to heaven?
I'm so extremely dissatisfied with it all. What worries me is that Castiel's exit felt like a write-off -- like, given their plans for season six, there wasn't room for him anymore, so they had to come up with a quick way to write him out without killing him, fan favorite that he is. I will be broken fucking hearted if that's the last we see of him. And if that was a write-off, should I be worried that Bobby's out too? Jim Beaver wasn't interested in taking a fulltime gig in season five when it was offered to him, so is the writing on the wall telling us he won't be back either?
Just. Fuck.