Regina's keys, which open any lock, have a skull engraved on the heads, making them literally and figuratively "skeleton keys."
Mr. Gold's pawnshop has several items from the Enchanted Forest, most recently the genie's lamp (though not the same from previous episodes).
Rumplestiltskin mocks David by saying "twoo wuv." This may be a reference to The Princess Bride (1987), in which the wedding officiant famously mispronounces several words, including "wuv, twoo wuv."
This is the first episode since Pilot (2011) that all regular cast members appear as both their Enchanted Forest and Storybrooke counterparts with the exception of Emma and Henry.