From Todd VanDerWerff's meditations on the season premiere:
I suspect when all is said and done that the history of Lost will cleave it pretty neatly into two different shows.
... 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) ...
But after the network set a firm end date for the show, it became something ever-so-slightly different. Gone were the long, meander-y episodes where we found out why Kate liked horses (and/or killed her dad) for the most part (there was one where we found out why Desmond says "brother" to everyone, but that was the last of an old era). The show became something much more purposeful, taking great strides forward in its narrative and starting to tie seemingly disconnected elements into a larger framework. In addition, the characters started behaving more like real people, no longer forced to do things they wouldn't do in real life in a similar situation by the constraints of a plot that said they couldn't because the show might run 10 seasons, and what would you do then? Most of the series' fans are deeply agnostic that Cuse and Lindelof really had a plan for how the series would run, but the episodes since that back half of season three seem to speak well for the two at least having SOME idea of how this was all going to play out. Plus, while there have been a few clunkers since the back half of season three (most notably season four's "Something Nice Back Home"), the series by and large has reinvigorated itself as one of the best hours of action-packed TV out there, flitting easily between genres, depending on who's got the episode focus that week.
If you like the show, read the whole thing. The division VanDerWerff outlines is real, I think, and the decision to set an end-date has played a big role - as predicted here - in saving the show from the wheel-spinning stagnation that defined most of its third season. For me, though, the real Lost divide will always be between the first two seasons and everything else, rather than between the pre- and post-deadline versions of the show. I'm part of the minority that actually liked the second season, hatch and all, and what I liked about it was the air of dread that still clung to the Island and everything about it - to the Smoke Monster and the Others, the cryptic numbers and the strange visions, the kidnapped children and the Dark Territory, the quarantine signs and the orientation films and all the rest of it. These things are still part of the show, in one sense or another (though many of them are part of plot strands that have been dropped, at least temporarily), but the dread started to leak away with the season-three revelation that the Others were just another bunch of squabbling, pretty-ordinary people with their own set of problems ... and now two seasons later it's all but gone. The show still hasn't explained "why," in Peter Suderman's memorable formulation, but in the course of explaining "what" and "how" it's lost the aura of barely-suppressed terror that clung to the Island in the first two seasons. Its mysteries are still real, but they've been domesticated: For all the apocalyptic overtones, I feel like the show partakes more of Michael Crichton, at this point, than Stephen King.
I like Crichton, of course, and I still like Lost enormously: Thanks to the late-in-Season-3 righting of the ship, it's still one of the best shows on TV, and hopefully will remain one to the end. But now it's a good action-packed sci-fi show, without the element of fear and trembling that kept me riveted through the first forty episodes or so. VanDerWerff misses the first two seasons' "simple moments of visual beauty" and "plot digressions that don't have to be tied into the master plot," and sometimes I do too. But more importantly I miss the dread.