The Candidates by Mastereh!?

The rules are that Smokey can't touch anyone who's a Candidate? It explains why he could kill Eko, Keamy's dudes, 815 Pilot, Nadine, etc... but couldn't kill Sawyer or Locke or Rousseau or any of the others on the candidacy list when it encountered them.

Also, if we're assuming the little kid is Jacob, then the fact that Sawyer could see him and Richard couldn't is probably a big indicator that only people that are candidates can see him.

And it also sheds light on why these people had to go back. Locke had to die to get the candidates back there. Locke had to die so that Jack would go back to the island willingly. Jin had to be stuck on the island so that Sun would come back to find him, willingly (remember how Christian appeared to Michael as soon as Jin left him and told him he could 'go now'? Michael's whole purpose was to make sure Jin didn't get on that helicopter).

The criteria to be a candidate would be that one must come to the island willingly. Miles was crossed off the list because it took money for him to come to the island. Juliet was crossed off the list because she said she wouldn't come unless her husband got hit by a bus. In order to be a candidate for protecting the island, the person must come out of their own free will/good nature/desire to return to the island.

Also, thinking about it, this sheds more light on Jacob interfering in their lives:

Jacob visits Kate as a kid; tells her not to steal; she disobeys, and is not a candidate. That is why her name/number were either crossed off or not on the wall.
Jacob visits Sawyer as a kid; sets him on his path to revenge; he follows that path willingly, and winds up on the Island
Jacob visits Sayid when Nadia dies; causes her death (because it would have made Sayid never want to come back to the Island); when he still won't come back, he goes to Ilana and says that he needs her help with something... bringing Sayid back to the Island? Of course, Sayid is claimed now, and probably not a candidate and since Jacob is dead Sayid's name was not scratched off yet.

Jacob visits Sun/Jin; they both crash on the island; marriage strengthens; child is conceived; Ji Yeon is born (Ji Yeon = Kwon) or perhaps both Jin and Sun are perfect candidates for remaining as guardians of the island for as long as they have each other they need nothing else.
Jacob's visit to Jack didn't really accomplish much... but Jacks visit to Locke is what ultimately leads to Locke dying and Jack feeling guilty and needing to come back to the island.

Jacob judges the future, MIB judges the past. Candidates are chosen to take over for them. They can appear as anyone who has died, however if they take over someone who was a candidate, they are trapped in their form for manipulating free will for their own accord.

Not everyone on the island is a candidate for leadership, but they may still be considered pure enough to reside on the island. These people are judged and allowed to live on the island without fear, EX. Cindy, Bernard, Richard, the Others. Either Kate or Frank are the sixth candidate for leadership.

The black rock was full of candidates. The french expedition was full of candidates. Oceanic 815 was full of candidates.

The end game: Paradise once again on Earth. Jacob is trying to save people, while MIB has lost faith in humanity; two sides and two perspectives to give balance. Both took over for Adam and Eve, but I'm guessing that Jacob was all too happy, and MIB did not want the responsibility or he was tricked into it. Both were originally normal men, and sacrificed a lot to become the eternal beings they now are.

Although originally he was innocent and kindhearted, Jacob has made him hate the world, and if MIB is able to leave the island and return home, he will likely cause the end of humanity, with no one to take over, and everyone fair game for him to judge or his leaving the island.