Clearly this was setting up a lot of things for later in the season, some of which made more sense than others. I don't have a whole lot to say about this one, but there are a couple things of note:
1) It took me a while to realize it, but it was really weird and new and exciting to see Dean struggling with the First Blade. Like, how many times have we seen Sam struggle to overcome something inside of him? But it's never happened with Dean before. The three times he's been possessed--the siren, the Khan worm, and the Civil War ghost--there was nothing to fight. It was BOOM, it's not Dean in there anymore. But here, he really had to struggle with what it was doing to him. In an ideal world, this would make him more empathetic towards Sam and all of the times he's had to fight something inside of him, but I don't know that we're there yet.
2) I really, really liked Sam having to call Dean back from whatever the blade was doing to him, because Sam knows what that's like better than anyone. I also really like tebtosca's theory that it was foreshadowing Sam saving Dean from the darkness of the Mark, finally getting to save Dean like he didn't get to from Purgatory (or from Hell, I might add).
3) Mini rant time. It's not just when it comes to Dean. For whatever reason, Show is completely incapable of showing us Sam's POV. We had multiple scenes with angry!Sam and Crowley, and I could think of tons of reasons for Sam's anger: Crowley's murderous spree last season, including Sarah; a reminder that Sam didn't get to finish the Trials and therefore demons are still walking the earth (because you know Sam puts that on himself and not Dean); a reminder that this is who Dean went to after Sam had to push him away; knowing that Crowley's the second demon to have been in his head and seen everything that's in there; or a reminder that the last demonic ally they had was killed by, oh, hey, Crowley, and trusting him would not be in their best interests. Did we find out which one of these was the right answer? No, we did not. We had Sam stomping around being angry for no given reason, when all it would have taken was a couple of lines of dialogue to present what Sam was thinking. What I wouldn't give for a little Sam POV this season. Just a little.
4) Whatever else is wrong with this season, at least we're getting lots of tied-up boys. And knifeplay. Mmm, mmm, good.