13. The Cork In The Heart
In S6 the Cork is shown as some piece of magic floating object in water which only the island's protector knows of. We're told it's the heart of the island and what keeps the island intact but nothing else.
11. About The Pregnant Women
Pregnant women on the island were seriously fudged by the 100th day. Claire was studied for this (before Aaron) and Juliet was brought on the island to figure this out. It never was.
8. Special Walt
Apparently Michael had special psychic powers but they were never explained or given any context or explanation whatsoever.
1. About Libby
Hurley's sort of girlfriend and one of the taileys. She was also revealed to be in the same institution as Hurley in his past. This was supposed to be a significant reveal but we never found out why because she disappeared in S2 along with the other taileys for no reason.