No idea if anyone else has noticed this but did you notice that the oceanic six weren't around when the hatch imploded? Sayid and Sun were on the boat and Jack, Kate and Hurley were with The Others. Theorising a little is this why only they left/can leave?
Aaron would be a problem there, but my friend suggested if you take the whole 'Time' and 'Contant' thing into consideration you can say that's why Aaron can leave, because everyone else needs a constant from their pre-island lives, but Aaron doesn't because he was born on the island.
Kinda holey but still something to note.