I'm sure ten people thought of this before me. . .and other people I've talked to have definitely added pieces that made it much better. The good parts are probably theirs. The bad parts that make no sense - those are all mine of course.
The Island is. . .a prison for the creature known as the smoke monster, aka MIB, aka Flocke.
Jacob is the jailer/guardian responsible for ensuring that this entity doesn't leave the island.
Jacob is allowed to pull people to the island (possibly from other dimensions) to aid in his mission.
Those people that he pulls there become The Others.
The Others goal is to ensure that MIB doesn't leave the island but MIB can take on different forms - can look like anyone.
When the plane crashed, even though they knew that Jacob was drawing some or all of those people there, they couldn't be sure who died in the crash.
Because the MIB can take on the form of anyone dead - anyone who had been in the plane crash could be the MIB as far as the Others knew - because they couldn't be sure that the original person didn't die in the crash - and get replaced by MIB.
It would follow that since anyone could in fact be the MIB, you wouldn't want to let anyone leave the island if it was your job to ensure that he stayed there.
That would explain why they went to such extreme measures to ensure the Losties were kept there - they couldn't let them leave in case the MIB had infiltrated somehow in his efforts to get home.
Yes, I'm sure this'll all be blown out of the water by next week. . .but this was my best guess as this point. :)
E.T.A. - It would also explain why the more they wanted to leave - the less the Others trusted them - because MIB's greatest desire is to leave the island - which ironically according to the definition Jack once applied to Juliet - makes MIB one of them.