1. The Smoke Monster
Island's protective system, left by long-lost ancient civilization. Or aliens.
2. Widmore v Ben
Widmore wants to get back to the island and to have it all to himself. Ben doesn't want Widmore to kill everyone. Something ugly probably happened between Widmore and the Others in the past, which led to Widmore being kicked out of the island and ever since he devoted his life to getting back and revenge.
3. Rousseau's Expedition
Found the island by incident.
4. The Numbers
AFAIR, the creators of the show said that the numbers don't mean anything, they are one of the ways to confuse the watcher.
5. The Four Toed Statue
Remnant of a long-lost civilization.
6. Richard
Either remnant of a long-lost civilization or someone who found the island by incident in the past. The island probably gives people inhabiting it either eternal or very-very long life, just like it heals people very fast.
7. Jacob
Same as Richard, only lived on the island much longer.
8. Christian Shephard
Island's way to communicate with the survivors.
9. Hurley Sees Dead People
Same as 8.
10. Time Travel
Writers' way to finally abandon all logical or sequential plot development.
11. The Purge
The Others got fed up with other people trying to exploit the island and killed them.
12. Walt
Same as 8.
Your turn.