"There are things that are known and things that are unknown; in between are doors." - - - Jim Morrison.
A good mystery is like a door.
The clues to what lies behind the door (the answers) can be found:
1) by the location of the door;
2) by the type of door;
3) the construction of the door;
4) the kind of latches or knobs;
5) whether it is locked or unlocked;
6) whether it has signage;
7) whether you can hear noises from inside.
Applied to the Island in LOST:
1) it was located in the Pacific Ocean, somewhere north of Australia and west of Fiji.
2) it was a tropical island
3) it contained ruins from past occupants
4) it was very hard to enter the island
5) it was hidden from searcher's view
6) it was so remote that it screamed danger to survivors that washed up on its shore
7) the noises of the smoke monster were loud and horrible.
So what was behind all the island mysteries?
Did we get a clear peak behind the door?
We know the island by its unique physics properties defies both time and space. But we do not know whether it exists in our current earth space-time, or phases in and out, or is another dimension (including but not limited to the afterlife).
We know that throughout the centuries, various people came to the island and built their own structures such as the Egyptian temple to the Dharma laboratories. But we do not know whether these civilizations were necessary to the development of the island or the saving of mankind.
We were told that no one entered the island without the permission of the island guardian. But it is unclear why Jacob would have allowed the US military to come place an atomic bomb on the island then leave it in the hands of power-mad people like Widmore. And if a guardian "needed" people on the island, why were those people's lives so meaningless and subject to frivolous deaths?
We think that the island's defense was the smoke monster and/or the guardian. We are not sure whether the guardian like Jacob was a smoke monster himself, or he had the power to create one. But since Jacob appeared to be immortal (until an unclear change made him give up the guardianship to Jack; perhaps the uncorking of the island), one could presume he was a supernatural being.
We also saw that when Rousseau's crew washed upon the island, the smoke monster was like an attack guard dog - - - killing and possessing them from the moment of landing. However, when the Flight 815 castaways landed on the island, the smoke monster's wrath was tempered to isolated incidents to Ben's "summoning" of the smoke monster by the water device to kill Widmore's men.
Even when the writers opened the door to the island, the interior is still quite dark. Even the answers are shrouded in mystery.