There's been some major discussion about what Zachariah pretty much told Dean that his destiny wasn't to stop Lilith, but rather as the angels allow the Apocalypse to take place Dean is supposed to face Lucifer himself. As this cold harsh truth to Dean, they are gazing up at the Josse Lieferinxe painting of St. Michael killing the Dragon, which indicates the prophecy that Dean will defeat Lucifer once the Apocalypse begins, as that is what has been foretold. Some have thought that this was a literal meaning that Dean would encompass the similar role of Michael, actually coming face-to-face with the beast and killing him in this holy war. However, I think this has several meanings, two in which I think are highly possible.
The Literal Meaning: Dean Will Defeat Lucifer
The fifth season will have Dean not becoming an embodiment of St. Michael or being possessed with some angelical power of sorts as some are predicting, but rather he will fulfill his destiny by becoming what I consider him to be the "second coming" of the series (which I'll come back to later at another time). I've always seen Dean as this christlike character, tragic and broken but has enough humanity in him to prevail through the thick and thin, and I think in order to defeat Lucifer he will use that humanity and strength through family and love to send Lucifer back to his eternal prison downstairs.
He of course cannot do that without help, for he is a mere mortal up against the Biggest Big Bad like ever, but if this will become true Dean will be the only one to defeat Lucifer in the end.
Another possibility is that instead of Dean carrying the weight of stopping Lucifer by himself as prophecised, is that it's both Sam and Dean that do so. Even though the divine prophecy says that the "righteous man that starts it must end it", technically both brothers started the Apocalypse. While Dean might've broken the first seal that brought on those chain of events, Sam also broke the last seal which unleashed Lucifer from his prison. So in those terms, they both brought on the end of the world, and in a sense that in the finale as they're brought together again, they'll have to find a way to stop the war and Lucifer together. Since we all know the Winchesters are better and stronger as a family, so I think that strength of love and family and humanity, along with other forces of course, is what will defeat Lucifer. Because remember that Lucifer hated humanity, and it would be a poetic justice if it's the power of humanity that will end him.
The Metaphorical Meaning: Personal Absolution
Dean has suffered through so much over the course of the series. They all have, the entire Winchester family, but it's mainly Dean that has carried all that weight and responsibility of keeping their family together, of protecting his brother, sacrifcing so much and getting very little in return. Season four introduced us the something more, Heaven and angels and the concept of having something to believe in, to have faith, is entirely new for Dean to accept. We've seen him struggle with his memories from his experience in Hell, and although he's not quite there yet we've seen the gradual shift and change within him over the course of the season.
What I'm thinking is that the painting signifies that Dean will conquer his own personal demons next season, in the midst of stopping the Apocalypse and dealing with the holy war and Lucifer. The image of Michael killing the serpent is a metaphor for Dean defeating his own demons, coming to his own realization of where he is and where he should be. The fact that Dean finally stepped it up in the second half of season four from wallowing in self-pity over what happened in Hell to finally coming into his own again, doesn't necessarily mean he's alright on the inside. He still needs to fully forgive himself, even Castiel says he sees within him, all his pain and anger and guilt. Dean have to forgive himself, he has to make peace with everything that's happened in his life, nobody can do that but himself. His journey next season will be to find that absolution, to find something to believe and have faith in, which will ultimately be his strength and core of fighting this battle against Lucifer (and again with the stength of humanity overpowering the powers of darkness).
Thoughts? Opinions? Am I being too far-fetched with my crazyass cracked out theories? I would love more input if you've got it, anything really. Keep in mind this is a mini-meta for what I have planned to go on for later, and more will indeed come. This is going to be one hell of a summer for meta-inducing stuff for my fandoms, believe me.