[D-Roc]

yeah, I also don't think "bad things" happen immediately..perhaps several months..

I think that it's to do with 'destiny' - as Locke once said, they were all brought to te island for a "reason". Many of them haven't fulfilled their destiny..their path..their "work". Hence I assume that the island will haunt them (as will their own guilt) until they return to complete whatever work they need to do.

I don't think it's about 'quota' per se, rather I think that they each have a destiny inside them..a destiny which is directly connected to the island. For instance we saw Alpert scouting young Locke in Cabin Fever..we also see Emily tell Locke that he was "part of a design" (Deus Ex Machina) - all of this alludes to the idea that Locke and the rest of the O6 are special in that their destiny's are interwined around the fate of the island. The island is both literally and metaphorically a 'magnet' in this sense.

Just my take on it.