Was supposed to be the series finale. Eric Kripke initially planned for Supernatural to be 5 seasons long. That is exactly why the episode is named "Swan Song" (goodbye)with a voice over and flashbacks. But Sera Gamble, the new show-runner, took over penning it from season 6 and on.
Stull Cemetary, where Dean found Lucifer and Michael, is an actual place about 5 miles west of Lawrence, KS. It has been rumored that in the past black masses were performed in the chapel ruins and that on Halloween night, a demon or devil sat in a tree inside the cemetery. It would supposedly answer questions about your future.
When Lucifer is showing Sam all of the people in his life who were really agents of Azazel watching over him, he refers to Sam's prom date Rachel. Dean is said to have slept with Sam's prom date Rachel Nave on prom night, while in his Djinn induced dream
Dean refers to Castiel as "Bukowski" after he suggests they consume copious amounts of alcohol and wait to die. Charles Bukowski was famous for writing novels and poetry dealing with alcoholism, death and many other depressing subjects.
Lucifer quotes the lyrics to the Charlie Daniel's song "Devil Went Down to Georgia" when he tells Sam, "a fiddle of gold against your soul says I'm better than you." The original line in the song is: "I'll bet a fiddle of gold against your soul 'cause I think I'm better than you."
When Dean says to Sam "you're a grown - overgrown - man", Jensen, messing around on set as the Supernatural actors tend to do, ad libbed the word "overgrown" in an attempt to make Jared (Sam) laugh. The ad lib made it into the final cut.
Sam, talking to Dean in the car, refers to jumping into the hole with Lucifer possessing him as a "Triple Lindy". This is a reference to the 80s hit film "Back To School" by Rodney Dangerfield. The Triple Lindy is an "impossible" dive performed by the main character at a dive meet, which helps the team to win the meet, saving them from what would have otherwise been a crushing defeat.
When Dean, Bobby, and Castiel are watching the news footage reporting worldwide earthquakes, the first shot showing a freeway on-ramp and overpass collapsing with dozens of vehicles on it is actually a scene from the movie Dante's Peak.
When Castiel, Bobby, and Dean are watching the news through the window of a store there is a sign on the window that says 'Big Deals', a possible shout out to the many demon deals on the course of the show; especially the one Sam made in the previous scene with Lucifer.
The trivia items below may give away important plot points.
While filming the scene in the graveyard after Dean gets beaten up, Jensen Ackles says he began to get emotional because it was an intense scene, but the tears began to dissolve the glue of his prosthetic swollen eye. It made his eye burn really badly, but they still had a while to go for filming, and he suffered through it.