Nobody will argue that Lost has had more than its fair share of problems through its run. Mysteries hyped up for months or years only to be explained in the vaguest possible sense, characters wandering off into the uncharted wilderness on a whim, and slews of new, uninteresting characters who generally only survive half a season or so. Much of this was rectified in the much-improved fourth season, which was fairly successful at trimming the fat and convincing viewers there really is a mythos behind all this.
But there's always something.
The main problematic subplot is the world's slowest-moving natural disaster, the love triangle between Jack, Kate and Sawyer. (Juliet should technically fit in there, but since the writers don't care about her, why should I?) The group, while being good actors for the most part, have absolutely no romantic chemistry whatsoever, yet the writers keep mashing their lips together like they have to share their oxygen supply. That it takes time that could be used watching a giant plume of smoke beat the living hell out of people makes it even harder to watch.
At the end of season 3, it looked like the whole mess had finally been resolved. Kate, finally tired of Jack's shit, had settled down for a nice, mutually abusive relationship with Sawyer. Jack had proven to have a thing for bland reproductive surgeons, shacking up with Juliet and driving Kate into the arms of another man. Or it might have been the other way around. Either way, everything was in its proper place.
Of course, this is Lost, where you can't assume a storyline is resolved even when every character involved is dead. On the island, things continued to look peachy -- Juliet stayed mostly off screen and Kate and Jack were too busy dealing with about twenty new characters to bemoan their imperfect love.
For the unaware, the first three seasons of Lost featured flashbacks in every episode about the characters' lives before the plane crash. These ranged from fascinating to completely worthless, but it was an interesting storytelling technique that generally helped keep the show from dragging everything out even further than they already were. For the fourth season, the creators mixed things up a bit by changing most of these into flashforwards, following seven characters that had found their way off the island under unexplained circumstances.
The flashforwards were the first sign that something was going horribly wrong. On the island, Jack and Kate were busy pretending they'd ever had a relationship, and it was great. But off the island, in the future, they were the most heartbrokenly estranged former lovers ever. Every other scene with the two of them involved him getting jealous over Sawyer, despite him having been left behind on the island years before, or him proudly declaring his love for her and wishing that they could get back together. When Jack got questioned in court over whether he loved Kate, they seemed to be pushing it a bit hard.
Sure enough, on the island, Kate and Juliet had a big tussle over his impromptu appendectomy and it was like nothing had changed, with one big difference. This time, we had been promised -- no, guaranteed -- that this would drag on for years to come. So come season 5, we can be treated to more scenes of them flipping out over their violently bland, passion-free relationship, knowing that it'll all end with Jack and Kate living happily ever after with lots of babies.
And the glacier slides on.