Charming mentions falling through a portal into Asgard. Josh Dallas, who plays Charming, also played Fandral in Thor (2011), who is from Asgard.
The ravine that Elsa crosses to find Anna is missing a bridge. This is because Regina destroyed it when instructing Emma in magic in The Jolly Roger (2014).
The Snow Queen tells Elsa that she's going to make a snowman, a nod to the song "Do you want to build a snowman" from Frozen (2013).
Elizabeth Mitchell (the Snow Queen) and Giancarlo Esposito (Sidney Glass) appear in one scene together. They most recently played adversaries in Revolution (2012).