By sheer co-incidence, given that we are so far behind the US in Supernatural and also slightly behind in Agents Of S.H.I.E.L.D., there were great tonal similarities between Meta Fiction and the most recent Agents Of S.H.I.E.L.D. episode, Afterlife.
High on chat, but low on action (although there was plenty of threat and aftermath), the primary purpose of Meta Fiction was to demonstrate to the Winchesters - and the audience - just how powerful the current Big Bad, Metatron (Moonlighting's Curtis Armstrong), is and how seemingly hopeless their struggle is.
While Sam and Dean captured Metatron's number two, Gadreel (Battlestar Galactica's Tahmoh Penikett), various parties were trying to convince Castiel that he should lead the angelic faction rebelling against Metatron's power grab for the role of God and the keys to the kingdom of Heaven.
The story took some surreal twists and turns, playing with Supernatural's idea of God being an author (as also - kind of - seen in the current season of Once Upon A Time), but while it remained a clever tool for messing with the protagonists' minds, and allowing for some 'meta' humour (such as talk of continuity errors), this wasn't Supernatural's strongest use of this gimmick.
As with the Agents Of S.H.I.E.L.D. episode, Afterlife, by the end of the episode it felt as though the main season plot had only advanced by the smallest of steps... but, in this case, it was a step of major importance.