Wow, it seems like forever since I last got my squee on! I hardly know where to start.
Remember when Castiel was perfectly happy for Sam to die? And was so disgusted by Sam's demon blood taint that he was reluctant to even touch Sam just to shake his hand? Wow, what a change. Now, even after everything that's happened, even knowing what Sam's destiny is, Castiel was willing to go toe-to-toe with an angel he's been friends (and maybe more?) with for millennia and even kill her to protect Sam. It shows, among other things, how much more confident and willing he is to think for himself.
I loved the image of Anna collapsing on the hood of the Firebird so that the wings fanned out behind her. Very cute.
Seeing John and Mary again was very emotional for Dean, but it was an exceptional experience for Sam. He was basically meeting both of his parents for the first time. Except for that brief moment in "Home" when her ghost saved him, Sam has no memories at all of Mary. As far as he was concerned, he was getting to talk to her and getting to know her for the first time. And although he grew up with John, Sam grew up with the obsessed post-fire John. This was the first time Sam got to meet the dad that Dean knew as a little boy before the fire.
I loved the scene of Sam with John and how Sam wound up defending his dad to his dad. In explaining to John how he hadn't understood his father until it was too late and how he hadn't been able to apologize, Sam was able to actually tell his father in a roundabout way that he was sorry, something that's been weighing on him since John died.
It's interesting that Michael, who is such a dedicated servant of God, doesn't really believe in the free will that God gave humans. Or does he? Was he just using the 'free will is an illusion because all of your choices lead to your destiny' argument to persuade Dean to say yes in the same manipulative and not-quite-lying way that Lucifer convinced Nick?
Once again it was confirmed that the suitability for being a vessel runs through bloodlines. So if Sam hadn't been given demon blood as a baby, would he be able to host Michael? Now that Dean has spent 40 years in hell, did any of it rub off on him enough that Dean could host Lucifer? And do the vessels for Michael and Lucifer, who are brothers, have to be brothers themselves? Is that why the YED didn't go after Mary's firstborn like everyone used to think he should have? Because Dean wouldn't have had a part to play as Michael's vessel unless he had a brother, and by that time he was too old, so baby Sammy got the demon blood? Inquiring minds want to know!
I loved the origin of the "angels are watching over you" line. The meaning of that phrase just keeps evolving. Michael wiped Mary and John's memories, so Mary apparently only subliminally remembers something about angels. When she told Dean that angels were watching over him, she meant it to be comforting. Instead, it eventually led to Dean's loss of faith because those angels who were supposed to be watching over him didn't save his mom. In the end, the phrase itself is far more sinister because angels weren't watching over him, just watching him and waiting for him to play his part in the apocalypse.
Oh, show, thank you for reminding me how much I love you.