If anyone had seen Jacob's ladder (movie with Tim Robbin's in it) the parallel's to LOST are disturbing.
I think the writers have intentionally left things vague so the viewership can put their own interpretations on things. However, season 6 and all the flashes are means for the characters to let go and be able to move on.
The big question is, is there really a mysterious island, a dharma initiative and so forth? The disturbing thought is that the last 6 seasons were merely the lost souls of flight 815 who could not pass on, they really did crash and sink to the bottom of the ocean.
The island was merely their way station to their next stopping place after death. So the whole thing was a merely a vehicle for that process and the island itself is not real.
Pretty interesting that the statue was on of the Egyptian god of the dead as were some of the inscriptions where Jacob was staying.
Why is it so hard to just leave the island? Because to leave you have to pass on to the next thing. Similarly to Jacob's ladder the passenger's via the flashes were able to see glimpses to how their life might have been had they not died on the plane crash.
The collection of souls represented those that couldn't accept that they were dead, they were together because of the shared traumatic event (although you could argue that it was all just Jack the story was about) and until they were ready to accept it they were LOST...