One thing also I should point out it showed the Ice King and Finn are closer than they thought: the Ice King was once human. As we've yet to see any other humans in Ooo (Susan it's still not clear), there's a greater connection between main protagonist and antagonist.

Also, is every penguin named Gunther? That makes me sad. Now how am I too tell which one was so evil Marceline's Dad couldn't even take his soul?