I'm really loving some of the IMDB posters' theories...here's a long and analytical one:
"It dawned on me at the very end; Ben was right.
So many times, we've questioned what all of these characters had in common. The answer seems pretty simple now; nobody has anything to return to off the island. Not one of the main characters has anything but a shattered life in the real world.
Let's take a quick look:
Jack - pretty obvious: he feels guilty for his father's death and most likely the deaths of everyone who didn't make it (survivors or others), his ex-wife still wants nothing to do with him, his nerves are shot (shaking hands looking at x-ray), and he's hooked on Oxycodine + is an alcoholic
Kate - fugitive, killed her father, mother wants nothing to do with her, is responsible for the death of the man she loved, may still be on the run
Sawyer - his sole purpose in life was to kill the man who pushed his father to kill himself and his mother. With this done, what is left for him? Every con he ever pulled was just to get himself closer to doing this.
Locke - his own father wanted nothing to do with him. He lost Helen. His job had no fulfillment. Not to mention that he was paralyzed.
Hurley - his bad luck was replaced by him being everyone's friend and living relatively happily. He no longer even wanted the money that he had. (As I see all these father connections with every single character now, I wonder if the future will see a falling out between Hurley and Cheech)
Jin/Sun - Sun was about to run away from Jin and disappear into America, Jin would have ended up trying to do the same to get away from Sun's father. They would have been starting over anyway. (While we're on the subject of father-connections, Jin figuratively killed his father by turning his back on him)
Rose/Bernard - Rose has terminal cancer and would be dying (supposedly) if not for the island's presumed healing powers (damn, no father connection here)
Claire - her father abandoned her (Christian Shepherd). Her baby's father also abandoned her. The only remnant of life which she was clinging to was flying to LA to give away her child. (seems pretty happy that didn't play out) (P.S. the psychic may have been right. Technically, Aaron was important and may be responsible for them being saved. If not for Aaron, Charlie may have never disabled the interference amongst other things for which the baby was the catalyst).
Sayidh - again, no father connection. I suppose he does have something to return to. He was still looking for Nadia (believed she was in LA), but that may have just been him clinging to a dream which is far long-gone. He betrayed his people (working with the US forces), so he probably couldn't return home.
Charlie - he was willing to sacrifice himself for Claire and Aaron. The number one thing that happened to him in his life was meeting Claire. His life had never been better than it was on the island."
Very true!