I suspect when all is said and done that the history of Lost will cleave it pretty neatly into two different shows.
There’s already been plenty written about how Lost’s two-hour fifth season premiere (which is really two episodes that probably could have been stitched together more neatly but most likely weren’t for syndication reasons) more overtly tugs the show into science fiction territory, while the stuff off the island with the Oceanic Six delves into the character-based side of the show that has kept it from having ratings so low it was canceled midway through its first season. But this divide between genre show and character drama is not specifically where the great divide falls for Lost. The great divide falls between the first half of the show’s third season and the last half of that season (which roughly matches up with when executive producers Damon Lindelof and Carlton Cuse convinced ABC to let them set a hard end date for the series). Before season three’s 13th episode, “The Man from Tallahassee,” the series was much more meandering and much more prone to fits of stupidity. But it was also a show with more time—time for things like visual poetry or narrative tangents that occasionally seemed like dead ends (fans hated season three’s “Tricia Tanaka Is Dead,” but it was really a fine little piece of television—it just didn’t advance the master narrative in any way). This series also was slowly shrugging off some of the pitfalls from first season, mostly set there via the original series conception by J.J. Abrams and Lindelof (Abrams has since left the series as an active creative force for the most part, enmeshing himself in Fringe, which actually is starting to feel a lot like Lost in some ways), and that could lead to some really ridiculous things like long flashbacks where we learned why Jack got a tattoo.