SEASON 5

Season 5 can be considered a watershed for LOST, or where the series fell off the rails, or foreshadowed the confusion to come in Season 6.

The preview poster claimed "Destiny Calls."

Destiny means that the events that will necessarily happen to a particular person or thing in the future, i.e.  she was unable to control her own destiny. It is the hidden power believed to control what will happen in the future; fate. Destiny can be considered predetermined outcome that an individual cannot control.

The main plot points for Season 5 took the initial island survival story and slammed dunked it in the ocean.

Season 5 centered mainly on the people left behind on the island and the series  "bad things" that happened after Ben turned the frozen wheel. Major plot points and themes included:

•  Time travel, the Island's unexplained erratic jumps through time, and whether it is possible to change the past or not, with the notion of "whatever happened, happened."
• Death, featuring three episodes heavily concerning themes of death, third parties statements about being dead,  and the apparent resurrection of Locke.
•   Leadership, especially concerning the history of the Others throughout both timelines;  the powerplay between Jack, Sawyer and DHARMA  in 1977 time travel arc, and the question of leadership of the Ajira 316 survivors.
• The O6 survivors'  journey back to the Island, following the lead and direction of mysterious Eloise.
• DHARMA Initiative ways of life, experiments and other activities on the Island, mainly Horace, Dr. Chang and Radzinsky trying to harness the island's "unique" properties. 
• Ancient Island history, including the tunnels, the smoke monster's alleged place of origin, the statue of Taweret, an ancient Egyptian god, and the conflict between Jacob and the Man in Black. 

 The time travel arc did not solve any of the mysteries set forth in the prior four seasons. It just added a supernatural component to an island that was not an island in any physical sense. Despite the producers adamant cries that the characters "were alive" the constant scripts telling the viewers that they characters were actually dead leading many to believe that their souls were trapped in a secondary universe, arguments continue to this day what was real and unreal. Leadership was a theme, but leadership itself lapsed into apathetic following, especially in the beach survivor's camp. Why anyone who got off the island would desperately want to return to their probable demise is still a crazy notion that none of the characters could adequately explain. DHARMA and its purpose got the short stick because we were introduced to new characters, but only partial and vague explanations of their work. It seemed like more tangent filler than real source materials. It was the ancient island history that held the most promise - - - with the detailed backgrounds of ancient Egyptian culture and rites could have been the Big explanation to the LOST mythology. But, despite the time and resources devoted to these plot elements, nothing substantial came of them.

One could say that LOST could have skipped Season 5 entirely and jumped from the end of Season 4 directly into Season 6 without that much more confusion.