At the end of the first season, the big mystery was the distress signal, which Sayid said has been repeating for more than 16 years. The distress call was made by Danielle after her shipwrecked science crew was wiped out by the smoke monster. It allegedly took out all the radio frequencies surrounding the island (which was false based upon Dharma's and the Others communications stations). The idea that a distress call was uniterrupted was to dash the hope of the survivors for the one thing they still were clinging onto: rescue.
At the end of the second season, Locke found a mysterious hatch cover in the jungle. He believed that it contained all the answers to the mysteries. His quest to open the hatch would later lead to tragedies. The hatch was created by another science team who came to the island to conduct experiments. One of the aspects of the hatch cover and its operation was The Numbers. The Numbers continued to pop up throughout various aspects of the story lines. We were led to believe that the key to solving LOST was in The Numbers. We would later find out that the numbers represented, if anything, candidates to replace the mysterious Jacob.
At the end of the third season, we had another element of the island show itself: The Others. Just as the survivors hoped was dashed when the first raft was burned by Walt, the second raft set sail for rescue. But before it got too far, it was intercepted by another boat of bearded men who kidnapped Walt, shot Sawyer, and destroyed the raft. The Others were the "native" inhabitants of the island, but we really do not know that for sure. The Others were supposed to be the headbutting rivals to the survivors' camp, but by the end they were mostly red shirt victims as were most of the 815 survivors.
At the end of the fourth season, we had the plot twist of the "flash forward," a peak into the future of the series with Jack screaming like a mountain man to Kate that they "had to go back" to the island. This shocking reveal meant that at least two survivors made it off the island. How?! We would later learn that a ill-fated Widmore assault team from a freighter would give the O6 an opportunity to leave the island via helicopter, to be rescued by Penny's boat, but not after several character sacrifices (Michael and Charlie dying, and Sawyer jumping from the helicopter). The whole O6 side story really did not amount to anything substantial and it was very forced to bring the characters back to the main action of the island. If you left a dangerous place run by a mad man (Ben), who in their right mind would want to go back? The people left behind were fellow survivors, but they could all be dead now. There was no reason for the O6 to return to the island.
At the end of season five, we have Juliet banging a rock on an atomic bomb casing yelling that it needs to explode. Many fans believe she detonated the device, but that did not happen. The incident was a massive electromagnetic discharge from the island itself (much like Desmond's fail safe key) which caused another implosion and realignment of island time. Juliet was a secondary character who was kidnapped by Ben to come to the island to fix the fertility issue of the island women. She wanted to leave but could not. She tried to manipulate men into doing her bidding, but was rarely successful. During the time skip to Dharma 1970s, she wound up undercover with Sawyer for three island years until the O6 returned to re-set the island. Juliet survived the implosion only to die in the arms of Sawyer in the beginning of the final season. Did her death actually matter in setting up the final season? No. Sawyer vowed revenge against Jack, but they never came to angry blows.
And the end of the lineal time of the series, we see Christian open the church doors and a white light engulfs everyone in the pews. This is supposed to represent the characters "moving on" in the after life, but we are left to imagine where they were going. Which in some respects is ironic, because we still really don't know where the characters have been.
So what were the key elements of the cliffhangers? An island that traps people, a hatch that controls mysterious energy force which may shroud the island from the world, the Others who don't want anyone to leave the island, the need of the island to have anyone who has escaped to return, that numerous deaths and sacrifices were needed on the island, and that in the end the island is given over to Hurley and Ben. They appears to be a need for the island to have a person "in charge." A normal island is merely a mountaintop of extruded volcanic rock that spears above the ocean water line. But this island is more, it appears aware.
A theme running through the various groups that landed on the island was that they were scientists: the Romans, the Egyptians, the Dharma group, Danielle's crew and various members of the 815 passenger list were highly educated professionals. So it seemed that the island needed "science" in order to do something it could not do on its own. And that may be the biggest cliffhanger of all: what did the island want?
