So assuming not all dead people automatically arrive there, what was unique about those dead people that made them show up on the island?
I've been giving this a bit more thought and might have to expand things a bit. (this might actually be worthy of a separate thread).
Basically you have:
1) `Special' or `Gifted' People - people with supernatural abilities, or the potential for such: Locke, Walt, Rose, and possibly Desmond and Hurly (because of the numbers). These people could probably reach the island on their own, and *might* be able to sneak others in with them. (a sort of `umbrela of protection'.
2) `Linked' people - people who have a connection with somebody already on the island. Examples here include Jack, Charlie, and Libby (all linked to Desmond). Another example would be Sayid (linked to Kelvin). `Linked' people have a good chance of reaching the island alive and remaining so, though this is far from certain. I suspect that there may have been a distant direct blood connection between some of the people aboard the `Black Rock' (african slaves) and at least one person aboard the drug smugglers plane (african criminals).
3) `Riders' basically people who are traveling with somebody who is `special' or `linked'. They might or might not survive arriving at the island (most of the passengers aboard flight 815 died, as did everybody aboard the drug smugglers plane). However, just making it to the island at all is enough to form a link between that person and somebody known to that person off the island. Juliette would have been a `rider' under this scheme, as would have most of the `minor' survivors of Flight 815.
4) `Accidentals' - people who reach the island via some sort of wierd fluke - not under the `umbrela of protection' provided by those who are `special' or `linked'. They basically reach the island dead, or die shortly thereafter - but again, just by reaching the island at all, that creates a `link' with people they know off the island.
Under this settup, the `islands' original discoverer(s) would almost certainly have to have been `gifted'.