Not sure what the main discussion is for the episode, but I think the main theme/takeaway was “You can only be you. What works for someone else will not work for you. And in this diversity, this originality, there is hope.”
We see this theme all through out the episode. Dean tries to be like Sam (his brother’s brand of “good”) and it fails. Dean can not “copy Sam” because he isn’t Sam. That’s why we got the “be yourself” lesson re: the pop culture reference of Elvis Presley. Elvis, before becoming popular, was turned down a lot for not fitting into one musical label. He didn’t sound like anyone else. He was different. People that hired/showcased musicians didn’t know what to do with him, a lot of times because they couldn’t neatly label him. Elvis sang a little bit of this and a little bit of that. He was part soldier, part actor, and part musician. He got to be many things, unlike Dean. And it is here that the show puts Dean’s focus re:fractured identity. Dean must become many things to overcome the Mark. He must accept and pursue every part of himself and then reconcile them all into one original identity.
Sam tells Dean he can copy Cain, but Cain’s story, while similar, is not Dean’s. Dean’s brother is alive. And while Cain’s past killed the love of his new life, this doesn’t have to be Dean’s fate. Cain never saw himself the way Colette did, just as Dean can’t forgive his own sins. Support is important, but it means nothing if you can’t acknowledge yourself as a foundation.