If one looks to a nexus point of the series, it is Australia.
Australia was a British penal colony. It was founded in order for the British to send the dregs of their society as far away as possible from their aristocratic society. Out of sight, out of mind. At the time, most of the prisoners would have died from the harsh voyage, disease or hunger. But that was the plan, to rid the criminals and their costs from the general, law abiding population.
Australia has been called The Land Down Under. It is a reference to its continental position on the planet in the Southern Hemisphere. It is also a clue to the harsh climate for most of the land mass and the tough conditions for the original settlers who thought they were sent to Hell.
The LOST characters all converged in Australia.
Ana Lucia came with Christian Shephard as his bodyguard after she murdered Jason McCormick and quit the LAPD. She had a grave sinful secret that she kept with her. She was told by her mother to return home and own up to what she did, but in the series she never came home because of the crash.
Arzt was meeting with his internet "girlfriend" but after his alleged girlfriend ditched him in a restaurant, he was returning home, depressed by the entire situation.
Bernard brought his terminally ill wife Rose to see Isaac, a faith healer near Uluru (Ayer's Rock), on their honeymoon. Bernard was looking for a miracle cure. This trick upset Rose greatly, for she had accepted her fate and she wanted Bernard to do the same. Bernard's plan put a uncomfortable wedge in their marital relationship.
Boone went to Sydney to "rescue" Shannon from her boyfriend. Boone and Shannon had an inferred sinful past coupled with the fact that Shannon was an indifferent spoiled brat who was suddenly left on her own. Boone went to help heal their strained relationship, but never had a chance to work it out.
Charlie was visiting his brother, Liam, in hopes of reuniting their band, Drive Shaft.However, he found his brother content, happy and sober with his new family; a family that Charlie dreamed about but never could have because of his addiction.
Cindy, as a flight attendant for Oceanic Airlines, worked in LA and Sydney as part of her route assignment. Viewers thought that she may have been an Other planted on the Flight since she quickly assimilated into the Other culture.
Claire lived in Sydney. She was going to travel to Los Angeles to give birth and supposedly deliver her baby to adoptive parents connected with Richard Malkin. She was giving up her child because her boyfriend dumped her because he could not handle the responsibility of fatherhood, that she caused her mother's coma state from an auto accident, and that she could not stand her biological father, Christian, who showed up in her life after the car accident.
Eko was investigating the miracle of Charlotte Malkin as a fake priest. He finished investigation and he returning with fake passport as the start of a new wave of criminal dealings.
Edward Mars working as a U.S. marshal, looking for fugitive Kate (which makes no sense since he has no jurisdiction in Australia). But he arrested Kate and he was traveling back with her.
Emma and Zack were two children on the flight who were traveling back to LA to meet their mother. We can only speculate that they may have been children of divorced parents and this was a custody transfer, not unlike Walt to Michael.
Gary Troup was an author on a promotional tour for his novel, Bad Twin; It was believed that he was traveling home with his fiancee, Cindy (the flight attendant) according to the non-canon Lost Experience.
Hurley was following a lead her got from Leonard Simms about The Numbers. He went looking for Sam Toomey, the man Lenny heard the Numbers from. Hurley found out from Sam;s widow that Sam was dead and that confirmed that the Numbers were a curse. Hurley was returning home for his mother's birthday.
Jack, after his mother, Margo begged him, went to Sydney to find for his father, Christian, and to bring him back home. Jack found his father dead from presumed alcohol poisoning which put Jack's mental state into an unstable mode as he had issues bringing his body home.
Jin was on an errand for Sun's father, delivering Rolex watches to the criminal bosses, Keamy, in Sydney and LA.
Kate fled the U.S. to avoid criminal prosecution to work on a rural farm owned by Ray Mullen, who would later turn her in for the reward. She was on the flight bound to LA to face justice for her crimes.
Libby is another great unknown. We do no know why she was in Australia, or why she was on Flight 815. She just appeared as a survivor from the Tail Section, even though we would later learn that she was a former mental patient with Hurley at Santa Rosa.
Locke, who was confined to a wheelchair, tried to force his way on a vacation "Walkabout" in the Outback. When he was refused passage by the agent, due to his disability, an angry and frustrated Locke had to return to LA.
Michael, on request from Brian Porter, his ex-wife's widower and adoptive father of Walt, went to pick up Walt, after his mother's death, so he could live with him. Porter was freaked out by Walt's special abilities. Walt had strong abandonment issues from all his so-called parents.
Nikki was an actress who had a role in a TV show. Paulo was her lover, aiding her in the con of her director paramour. They stole the director's cache of diamonds and were fleeing to the U.S. to start a new life together.
Rose thought she was on her belated honeymoon with Bernard, but became upset when she found out that it was a trick for her to see a faith healer.
Sawyer was tracking down a lead on Anthony Cooper, the man who "killed" his family. Using the information from a fellow con man, Hibbs, he found the man he thought was Sawyer and killed him. But that was a mistake, a con; it was hit job for Hibbs. Distraught and drunk, Sawyer got into a bar fight which led to his expulsion.
Sayid was recruited by CIA agents Alyssa Cole and Robbie Hewitt to infiltrate a terrorist cell in Sydney by turn his old friend turned terrorist, Essam Tasir, in exchange for information on his love, Nadia. Sayid could not go through with the plan, but his friend died anyway. He stayed an extra day in Sydney to bury his friend, then he was going to LA to search for Nadia.
Scott had won a sales prize at work: two week Australian vacation, all expenses paid.He was returning home on Flight 815.
Shannon was living in Sydney with her abusive boyfriend. Having lost her meal ticket, she was going to con Boone into giving her money and a new place back in the U.S.
Sun was going to ditch Jin in the airport in order to runaway from her marriage and her father to start a new life, but at the last moment she changed her mind and accompanying Jin to L.A. on Flight 815.
Vincent was Brian Porter's dog. He was given to Walt; they traveling to U.S. to live with him and Michael.
Walt lived in Sydney with his mother, Susan Lloyd, and adoptive father, Brian Porter. When his mother passed away, Brian freaked out with the responsibility of raising Walt so he got Michael to take him (which legally made no sense.)
Most every character in the series converged in Sydney with deep, dark, haunting secrets, sins and personal issues that could overcome the collective psyche of an entire plane of passengers on their long voyage home. There is a thought that there is a power of positive thinking. If so, then there would also be a power of negative thinking. So many characters on the plane had negative thoughts, fears, and mental issues - - - could that psychic power brought down Flight 815 into a spiritual dream world of the Island? Or were these negative thoughts and sins the mere sorting device that teleported the "survivors" into the bowels of Hell for punishment or purification or redemption to find a path to the sideways church after life?