Just some thoughts I had after rewatching, and reading other peoples' posts...
1. Turns out my belief that Cain had no children was quite wrong. However, it does seem that all of his descendants were killed in the Flood, so he shouldn't have had any of them around today. But whether he was killing those in his own line, or those related to his "issue" as the Father of Murder, one important thing that didn't come up is that if Dean is on his list, so is Sam.
2. When I wrote my review, I said I wasn't sure why Dean confessed that he'd lied to Crowley, and that I thought he was going to kill him when he hesitated to give up the Blade. But while writing my drabble for this episode, when I was thinking about how Dean cut Crowley with words instead of with the Blade, it came to me (as I know it came to others already, but have pity on me, I'm slow). He can't kill Crowley. In Cain's prediction, Crowley is the first domino. It's Crowley and then Cas and then Sam. So Dean couldn't kill him even if he wanted to, but he had to get rid of him. Had to eliminate the temptation.
3. We didn't actually see Cain die. On first watch, I assumed they were just sparing the expense of the cheesy Abaddon death SFX (and I was grateful). But
cassiopeia7 has some commenters who believe he's not actually dead (her post is f-locked, but basically, some people think that because we didn't see the death or a body, and because Dean healed instantly when he cut himself with the Blade, it wouldn't have killed Cain). And this is something I'd like very much to believe, because damn. Cain. More of that, please and thank you. But on rewatch, I'm pretty sure he's dead. We didn't see it, but we certainly heard the Blade enter his body. And if Dean is aware of #1, there's no way he would leave Cain alive.
4. I've been kind of hung up on Dean's immortality lately. But what if he's not? Why do we think he's immortal in the first place? Did Show explicitly say that, or did we just make that assumption based on Cain's immortality? Because Cain is a demon, not a human, so he's not going to die of natural causes. And the canon (Biblical, that is) Mark of Cain, from what I've been able to find, didn't make Cain immortal. It just prevented anyone from killing him. So maybe Dean's simply immune to being murdered, but not immortal.
5. Dean's little smile, when he heads up the stairs to face Cain, reminds me of his little smile when the clock strikes midnight in No Rest for the Wicked. That brave little I'm gonna die, but you keep up the good fight, Sam face. And I already pointed out how the pre-fight discussion reminds me of Swan Song.
6. This might be one of those episodes that inspires some truly fantastic fics. Off the top of my head, I'm going to suggest you read
amberdreams's gorgeous "double drabble,"
Bipolar,, and
frozen_delight's heartwrenching and beautiful and amazing coda,
Swimming. No spoilers in the comments, please! :-)