Over on Geoff's blog, he writes that the show will pull through in Season 6 because it always puts the characters first and the mythology second. But I think that this season is among LOST's weakest precisely because that wasn't true.
The Jack-Kate relationship would serve as Exhibit A, I think. Jack became thoroughly unlikable and his skepticism transformed into a cynicism that bordered on nihilism; Kate was present but virtually inaccessible with respect to her feelings and motivation. The two interacted so little on the island that it wasn't until the finale that I realized they still had feelings for each other and we were supposed to be rooting for them.
Conversely, Sawyer and Juliet was pulled off in a surprisingly convincing manner. Still, given that their relationship arrived fully-formed, a flashback covering the previous three years on the island and giving us a reason to feel invested in them as a couple would've been nice. We're supposed to sympathize with Juliet, I think, when she's made to feel anxious by Kate's arrival, but instead I felt a sense of inevitability - that Juliet would be become jealous and do something dumb, that Sawyer would give her a reason, that the Jack-Kate-Sawyer triangle would reassert itself.
For all the character work that didn't happen, then, it's also surprising that so little of the mythology seems to have been developed or resolved. The trip into the past added surprisingly little to our understanding of the Island and its inhabitants, while the much more interesting stuff happening in the present was barely given any time to breathe. When you toss in the introduction of some timeless battle between Jacob and his adversary (now known to the internet as Esau) and a whole new group of survivors, it's enough to make one suspect that there will be little more effort put on the characters next year.
This is not to say that I think the show went all wrong. Some individual episodes around the middle of the season were fantastic - Locke's, in particular - and the season really picked up when it seemed they realized that it should have started with Jack waking up on the island and not spun its wheels for several episodes trying to explain how he would get there. And the inversion of Ben and Locke's roles - the evil schemer and naive follower - was damn clever and fiendishly executed. (These two are probably my favorite characters, and no less so now that Ben is emotionally crushed and Locke is no longer Locke.) And while Sawyer's turn as leader and thinker was far too short lived (again, this development would have benefited from a flashback), I liked what they did with him, even if they pissed it away with a predictable reversion in the last couple episodes.
Finally, I like that I have absolutely no idea where this next season will start. (I guessed that Season Four would start where, it turns out, Season Five began, and that Five would open with them back on the Island.) Having them landing in LA in 2004 would be a ballsy move, but I'm not sure where they would go from there; picking up in 2007 with the characters having landed in LA three years earlier would at least fit the timeline that the show has established, but would seem to make equally little sense. Really, though, nothing to this point suggests that Daniel was right in thinking that the past could be changed, and so I wouldn't be surprised to see the Losties from 1977 end up in 2007, as little sense as that might make. (The white flash did, after all, share a certain resemblance with the white flashes that sent them flying through time earlier in the season. In which case we have no reason to think that Juliet's necessarily dead.)
But that would just be a really cheap way to get them back, wouldn't it? And make it seem as if they were in the 70s for no good reason, except maybe to explain... no, they didn't even really explain or show us anything that we didn't already know or suspect we knew. (Radzinsky was more intriguing as a stain, and DHARMA more interesting when everything we knew about them was gleaned from old film and a pile of corpses in the jungle. And we discovered absolutely nothing new about The Others, except that Ellie and Charles have some sort of 'complicated' relationship, both romantically and with respect to the leadership of the group. Though we don't know what that complication is. Still.)