I think the 'plane is down, and everyone is dead' is somewhat of a red herring, trying to lead us to think that the island is purgatory (which is one fan theory that the creators have definitely discounted).
Maybe what happened is that for some reason, all the people on the plane were intended to end up on the island, and whoever planned it also planned to have another plane crash and be found, identified as flight (whatever)? Of course, who planned it is anyone's guess.
I wonder if they are in an alternate dimension? So the plane crashed because it breached some kind of dimensional barrier? (science fiction). That could explain the 4 toes on the statue.
(But sometimes, I wonder if the creators/writers even have a 'bible' they are following, and really know what the big picture is, or if they just make things up as they go along? I know they add characters that they didn't plan for in the beginning, like Mr. Eko.)
I have absolutely no idea, but it's really difficult to sort out the bad guys from the good guys. The Others claim to be good, but would they force someone to kill his father (no matter how bad the latter was) if they were? Is Julia a 'good person' who is an innocent victim? Or is she bad? It seems like everyone on the plane has done something bad in the past. And they all seem to have ties to each other (or at least everyone has ties to at least one other person on the plane).
I think "Lost" is a very appropriate title for the show, though! since that's how I feel at times.
I hope they don't end with it all being a figment of Walter's imagination. That would be a rather cheap trick.