MY THEORY:
We'll find the smoke monster is, in fact, a titanic fart that has escaped Hurley's ass. The alternate realities are all methane-induced hallucinations. The "candidates" are people that ate the same combination of foods (we can assume prawns, vegemite, and Fosters) before getting onto the plane. They have the promise of contributing to the power of the smoke monster, or keeping him at bay through force of individual will and sphincter control.
The smoke monster's primal need to get off the island is a physical property of gas expansion (Boyles law?). The damage that people fear but have no proof he will unlease on the rest of the world is due to the fact that he is a colossal and smelly fart. The beauty is that the two camps have the same goal. One camp wishes to immediately release the Black Smoke from the island, the other is trying to contain him through their internal efforts in the hope that he will fade away.
In this light, the exploding tail section, whispering breeze-like voices, the mysterious hatch, people going down wells and through tunnels, etc. become fairly obvious metaphors.
The eternal struggle of good v. evil can be boiled down to the decision of whether to let fly on a plane or not.
