Things go from bad to worse in this episode, as the pressure mounts and the effect of the time travel becomes lethal on the island. In very general terms, it's now known why the time shifting is taking place. For some unknown reason, related to Jacob's commandment to Locke in the fourth season, Ben's decision to move the island in Locke's place brought about the uncontrolled jumps through time.
Considering how important the notion of "destiny" has been and continues to be, perhaps that shouldn't come as a shock. The rules of time travel (as shown in "Flashes Before Your Eyes", "The Constant", and this season) demonstrate how little free will matters in the long run. Everything happens the way it's supposed to happen, and free will operates within the relatively minor confines of quantum probability.
Jacob's claim that Ben wasn't supposed to be the one to turn the wheel may not be entirely accurate. It may be true that he didn't want Ben to be the one to do it, but the fact remains that everything is happening as it has already happened. Charlotte's story in this episode is a clear indication of this. Alternatively, all of this might have still happened in some manner had Locke turned the wheel the first time, but the uncontrolled time shifts were always part of "history". (That said, this is hardly the first time Ben took on a role that was meant for Locke.)
The accelerating time shifts are a bit disappointing, because it would have been good to see more detail about the fate of the Science Expedition. In broad strokes, it fits the information that Danielle provided to Sayid and Hurley back in the first season (the music box, Montand's arm, her decision to kill the rest of her crew, the circumstances of her confrontation with Robert). There are hints that exposure to the inner workings of the Cerberus system brought about the effects that Danielle later called "the sickness".
More detail would have been nice, but this does present some interesting new information. Montand's experience is very similar to the one Locke had at the end of the first season. This particular "Cerberus vent" was guarding a location called "The Temple", which is where Ben sent Richard at the end of the third season. The Temple has markings very similar to Egyptian hieroglyphics.
Even Charlotte's fate manages to connect some of the dots. She also mentions knowing all about ancient Carthage, which just happened to be located relatively close to the location where Charlotte found the remains of the polar bear and Dharma references, and where Ben appeared after he turned the Magical Donkey Wheel. Could this perhaps point to the age of the Others' civilization? Did someone from the island found Carthage?
Back in 2008, Ben's plan to gather the Oceanic 6 and Friends has gone horribly awry. That's not much of a shock, since they all have reason to hate him, but it was a little surprising that Jack sat back and kept mum. Wasn't he convinced that they had to go back to the island, even before Ben came around? Wasn't that the entire point of the third season finale, to show Jack's resolve? It seems odd that he would suddenly keep quiet and let the whole plan fall apart. Getting Kate, Sayid, and Hurley to cooperate is going to be quite the challenge.
Despite the dense nature of the episode, this is really all about Jin and Sun, and the episode works on that level. Jin's experience with Danielle, and his observation of Daniel and Charlotte, reminded him of everything that's wrong with the island and the danger it would represent to his family. Whether Locke was planning to betray Jin or not, the cruel irony is that his wedding ring, meant as proof of his death, becomes the very thing that convinces Sun that Jin is alive. The character focus is subtle, but it was definitely there.