It's a fairly common storytelling device within the genre: take a character into the future, show how bad it will turn out to me, and then return the character back into normal time. Not only does it reveal something about the character in terms of his or her reaction to the revelation, but it increases the tension for the audience. Knowing how bad it could get gives the audience a reason to invest in the characters and their attempt to prevent it.
In this case, we don't really know if this is a true vision of the future. Zachariah has been known to mess with Dean's head before, and manipulation is the order of the day. Zachariah doesn't seem sincere even when there's plenty of reason for him to be sincere. Dean catches on to that pretty quickly, and does his best to make a choice that might change things up a bit. Of course, because Future Dean said that Dean would do exactly that, the audience is left with that familiar tension. Has that future been avoided, or will this just be a much harder road to the same bitter end?
A lot of people questioned whether or not the Brothers Winchester should work together before they resolve their serious individual issues. I also would have preferred that the separation last a bit longer. It would have given Dean time to find himself again, to get back a sense of self-reliance. That journey could have also given him perspective on his weaknesses, driving him to an internal resolution to those problems. As entertaining and dark as this episode was, it was really a way for the writers to cheat and get Dean to that self-realization a lot faster.
Future Dean was a much harder and less humane individual, willing to sacrifice his friends and family to win the day. But at the same time, it was also a version of Dean that was all too easy to recognize. Dean has shown a capacity to set aside human compassion for the sake of the mission in the past, and Sam was an empathetic influence on Dean. It's enormous progress for Dean to recognize that flaw within himself and understand that Sam was the key to avoiding that loss of perspective.
For all that this vision of a possible future was able to jump start Dean's self-awareness trip, it didn't do much for Sam's side of the coin, and that is probably the biggest downside. Sam needed to struggle on his own for a lot longer. He's still running from the devil, but I don't see that his personal issues have been addressed at all. Granted, it's not as if the character is now confident, as if that journey had taken place already. Sam is still conflicted and adrift, unsure what to do to make amends.
It all comes down to one vital question: where are the writers going with Sam? Dean is fairly well-defined now; he has a vision of the future to inform his choices, and Castiel is there to help him. But what about Sam? Sam is running from the devil and himself. He's reacting, not acting. Something needs to break to get Sam back to his strengths.
That uncertainty plays into the final scene of the episode, which felt incredibly awkward. Sam's lack of confidence is palpable, and he really doesn't seem like himself at all. For that reason, an alternative interpretation comes to mind. What if Sam has already given in to Lucifer? What if the oddness of the scene was intentional, meant to convey that Lucifer is still working out how to impersonate Sam to achieve some short-term endgame?
It would be a shame to ignore the visions of the future, because there was some great continuity there. I'm sure that it was just a way to resolve a loose end from the second season, but the shout-out to the Croatoan virus was a nice touch. It further underscores the notion that all the twists and turns over the years have been leading to this, and that Sam really was being set up to be the Antichrist Superstar so he could unleash Lucifer and become his vessel.
I thought the portrayal of Lucifer was very well done, and extremely consistent with what had been shown before. It's important that Lucifer have a recognizable motivation, and there's plenty of room in the classical interpretations of Lucifer to develop a consistent version for Supernatural.
I like the idea that Lucifer set the whole thing in motion out of his idea of love. All his actions against humanity, right down to the creation of demons from human souls, has been to prove just how right he was to deny humanity its appointed place as God's favorites. Demons, from his perspective, would simply be the true face of humanity laid bare. The scope of Lucifer's hatred for humanity rightfully informs the depth of his self-delusion.
Some will still believe that the writers are framing Lucifer in a sympathetic way, but considering how quickly Dean calls him out on how twisted his logic is, that's clearly not the intent. Instead, indirectly, it demonstrates how the rest of the angels have fallen. They have been saying exactly the same things that Lucifer says in this episode. Only Castiel seems to be staying the course. That makes it very clear that the writers aren't trying to say that angels are inherently evil, but rather, that they have fallen without realizing that's what they've done.
This is another strong episode for Misha Collins. Future Castiel was definitely played for chuckles at first, but it's not left there. Castiel is not praised for his change in lifestyle; he's painted as tragic for having lost himself. He's a total mess, and he knows it. Seen in contrast with everything that Lucifer says, it is a chilling reminder that there may be a reason why humanity deserves what it might get. The Brothers Winchester don't just have to save humanity; they need to show why humanity deserves to be saved in the first place.