Originally Posted by Jari
Michael was trapped as a whisper because of his actions (killing the innocent Ana-Lucia and Libby in S2). Sayid, Sun and Jin didn't have the same 'sins' as Michael. I'm not sure where you are in the series so I don't want to say anymore than that.
I know I'm wasting my time saying this because people who are still saying that there'd be a scientific explanation or that they made it all up as they went along aren't going to stop thinking that. Anyways, I'm very familiar with the show and there are pieces laid out sometimes years in advance so they had some idea of where they were going. Which makes more sense, that they knew the basic answers to the mysteries that they introduced and how it worked with what came before or that they chose to make it up as they went so it would be much harder to pull it all together and answer? Why would they make it harder on themselves especially once they had a date to end the show and knew how long they had to keep the story going? And I still have never seen a quote from any writer on the show say that there would be a scientific explanation (probably because it's obvious that there is no scientific explantion for a magic time-skipping island).