A cliffhanger is a story telling device which stops the story at a precarious, dangerous or surprising stop until the next chapter or episode is revealed to answer or explain the prior plot point.
LOST contained numerous cliffhangers, some from episode to episode, some from season to season.
In examining the end-of-season cliffhangers, and the promise of their resolution, reveals the following:
Season 1: The opening of The Hatch and the attack on the raft which strange people kidnapped Walt.
Season 2: Jack, Kate and Sawyer are taken by the Others; Locke lets the Hatch timer run out to a system failure (which aftermath is in doubt); and a listening post tells Penny it has found the island.
Season 3: Jack's flashbacks are revealed at the airport to be flash forwards, that Kate and Jack will escape the island.
Season 4: The island moves and the man in the coffin is revealed as Locke (but not how he died).
Season 5: Juliet is trapped in the construction pit, left to pound on rock on the atomic bomb.
Season 6: The cast gathers in the sideways church to be engulfed by a white light.
We would learn that the Hatch was a Dharma science station, that had to be manned by a person who needed to input The Numbers (for some unknown reason). We would learn that a group of native inhabitants called The Others were behind Walt's kidnapping and the sinking of the raft. The Others claim to be "the good guys" but state that the island is their domain.
We would learn that Jack, Kate and Sawyer were captured by the Others so their leader, Ben, could manipulate them to apparently serve the goals of a mysterious person named Jacob. Jack was needed to perform spinal surgery on Ben. Sawyer was needed to be conned by Ben to quell any rebellious notions that the survivors would have in the future, Kate was used as the romantic pawn to keep both Jack and Sawyer in line.
We would see that if the Hatch timer went to zero, an alarm would signal, and hieroglyphs would appear that said "He Escapes Place of Death," and there would be a massive surge of magnetic energy that would rattle the entire island.
We would learn that Penny had hired men to listen for the island so she could find its location. We were left wondering why people could not find the island.
We saw the sudden plot inversion to the flash forward with Jack and Kate at the airport. We saw an upset Jack losing his nerve. When Kate left, saying "he is waiting," we did not know who she was talking about (Sawyer, Ben or some other person). The question was how did they get off the island, and what happened to them.
We learned that several 815 survivors got on board the helicopter from Widmore's freighter just before it blew up. But running low on fuel, Frank attempted to return to the island when it suddenly vanished without barely an ocean ripple, stunning everyone on board.
In another flash forward mystery from that season, the man inside the coffin was revealed to be Locke, but we did not know how he died until it was revealed that Ben killed him.
We would learn that Juliet "survived" the construction site implosion (meaning that the atomic bomb did not detonate), but she mumbled that "it worked" (but we don't know what that means). She is embraced by a time skipping partner, Sawyer, who holds her when she dies. He vows revenge against Jack for his foolish plan that led to Juliet's death.
In the end, we see the smiling faces of the main characters engulfed by a white light as Christian opens the church doors, but we don't know what that means or where the characters are "moving on" to.
The answers to the cliffhangers were mere isolated facts, little puzzle pieces, which did not connect together to form a clear picture to solve the biggest mysteries of the series.