Spoilers, natch.
That ending just made me laugh, because both Sam and Dean have been to hell, and to heaven, so it stads to reason that the only place left to send one of them is Purgatory. Though if it leads to more Cas next season, I'm all for it. I've missed him.
Poor Sam, though, totally alone now. I'm kind of hoping he somehow rescues Meg to help him rescue Dean and Cas, though, because (a) I like Meg now! That actress is awesome and (b)I like Sam with a demonic girlfriend, what can I say. (I was one of the 3 true Sam/Ruby shippers back in the day). Also, maybe this could include bonus Kevin-rescuing, since he's kind of cute.
It's funny, I cried so hard when Bobby died mid-season, but this time it barely affected me. I think they kind of misused the vengeful spirit storyline--they might at least have made it pivotal to the Leviathan plot, or made the need to sacrifice him more dramatic, or something. On the other hand, I wouldn't be surprised if he turned up in Purgatory. There has to be a penalty for ghosts that don't go with their Reapers, right? Plus he had basically turned into a vengeful spirit.
I have a great deal of love for crazy Cas and his fixation on bees and board games, though I suppose being hunted by every monster ever is going to restore him to sanity quickly. (And I was so excited that they played Vincent to illustrate his emo! The first concert I ever went to was Don McLean, trufax.)
I won't miss the Leviathan subplot per se, but I will miss Dick--he's definitely one of my favorite villains.
I was honestly expecting Crowley to bite it in this episode (and maybe have Meg become Queen of Hell), but it's good that he's around--they definitely need a good big bad next season once Dean and Cas are sprung from Purgatory (and I can't imagine they'll be there more than half the season), and he definitely is not happy with the boys or Castiel.