A number of people had predicted much of what is revealed in this episode, so I suppose they should be congratulated for their insight. (Either that, or they read the spoilers.) I was hoping that the story wouldn't take that particular direction, but I suppose that it makes sense within the premise of the series. This is not about angels vs. demons; it's about the Brothers Winchester. And that's the point made throughout the episode, first by Bobby, and then by Dean.
A lot of people are going to object to the characterization of the angels, and the notion that the angels would want to bring about the apocalypse and release Lucifer. Making the angels no better than the demons is a bit dodgy. It's consistent with the characterization of some of the angels, who had little concern for humanity as a whole or the cost of the coming war to the human population, but it still seems a bit too much to have Zachariah tempting Dean with beer, food, and virgins.
Yet I imagine that an angel might fall without realizing that's what they've done, and that it could happen on a fairly widespread basis if that is part of the master plan. It occurred to me when Zachariah said that "God has left the building" that it may only seem that way from his point of view. It may be that God wanted the angels to feel that way, because the resolution to the conflict was always meant to be in the hands of humanity.
Normally, when something so wrong is revealed, it means that it will somehow be circumvented. When Castiel was convinced that Dean was right, and that the "plan" should be rejected and stopped, there was every reason to believe that Dean would arrive in time. That's what made it so crushing that Sam couldn't be stopped. Sammy had no idea that his best intentions were literally going to open the road to Hell.
I'm not sure whether or not all the details of the mythology support the notion that Azazel's plan was to facilitate the rise of Lucifer. This episode implies that the Antichrist Superstar competition was designed to produce the most promising candidate to break the final seal, but wasn't he producing children with demonic blood long before 1972? That didn't quite track for me, even though I love the idea that all of these continuity elements are finally coming together.
It also seems a bit too easy for Ruby to be working for Lilith. I liked it better when she seemed to have an agenda of her own, apart from the plan to release Lucifer. That said, I think her time had come. Ruby served her purpose as the temptation for Sam, drawing him to the dark side. Besides, I don't buy this claim that Sam chose the darkness based on his own choices, however; the demon blood clearly had a drug-like effect, given that he suffered from withdrawal.
I'll give credit to the writers for going much further than I had anticipated. I was sure that Sam would prevent the breaking of the final seal, and then Dean would be forced to bring Sam back from the brink. I never anticipated that Lucifer would actually be released. Frankly, if they are going full out apocalyptic, I don't see how they're going to pull it off well on their current budget. Hopefully the final season of "Supernatural" won't be an example of biting off more than one can chew.