Emily Nussbaum ponders it:
Narrowly defined, a cliffhanger is a climax cracked in half: the bomb ticks, the screen goes black. A lady wriggles on train tracks—will anyone save her? Italics on a black screen: "To be continued . . ." More broadly, it’s any strong dose of "What happens next?," the question that hovers in the black space between episodes. In the digital age, that gap is an accordion: it might be a week or eight months; it may arrive at the end of an episode or as a season finale or in the second before a click on "next." Cliffhangers are the point when the audience decides to keep buying—when, as the cinema-studies scholar Scott Higgins puts it, "curiosity is converted into a commercial transaction."
The Dish 