In no particular order.....
1: Walts powers and connection to the island
2: The ageless Richard Alpert
3: The Monster
4: The statue
5: Where the kidnapped survivors are
6: The incident
7: Jacob (need I really say more)
8: the black dust around Jacob's cabin
9: Libby's connection to Des and Hurley
10: Ben and Charles' feud
11: Eloise and the lampost
12: The whispers
13: The Black Rock
14: The Numbers
15: Adam and Eve
16: The temple
17: Claire's whereabouts
18: Christian and Locke's rebirth's
19: Henry Gale (the real one)
20: How Danielle survived the purge
I know I missed some but I tried to keep it at 20.