Beyond the structure of the story going forward, there were plenty of interesting tidbits. It has now been confirmed that Jacob's rival and Cerberus, the"smoke monster", are one and the same. This was essentially predicted in the review for "The Incident". This is consistent with the notion that Cerberus only took the corporeal form of those who had died. It is also consistent with the interpretation that the mural from "Dead is Dead" pertains to Anubis (Jacob) and Cerberus (Jacob's rival), in apparent opposition.
But it does raise a different question. If Jacob's rival was taking the form of Jack's father, and Jack's father was apparently trapped in the cabin, that suggests that the ring of ash around the cabin was intended to keep Jacob's rival trapped there. If that was the case, how would Jacob's rival have roamed the island as Cerberus, the "security system"? Why wouldn't he have been confined, especially since he was clearly unable to cross the ashes in this episode?
Jacob's rival states that his goal is to get home. The key questions are where, how, and why. It's possible that "home" is the Temple, since the Others (the ones Ben sent there before the third season finale, at the very least) were preparing to keep Jacob's rival out by various means. But Jacob's rival seemed to come out of its underground vent in the outskirts of the Temple. It explains why Danielle and others thought of Cerberus as a "security system" for the Temple, but it doesn't explain why Jacob's rival would be so closely linked to the Temple if that was his final goal. Instead, it seems likely that the constant connections to Egyptian myth and culture are a clue. The time/space anomaly in the Orchid connected to Tunisia.
Using the form of Locke certainly pertains to the "how", as well as killing Jacob. Since taking on Locke's form is not appreciably different than taking on any other dead person's form as Cerberus, Jacob's death must change something that will allow Jacob's rival to do something he couldn't do before Jacob's death.
That may have something to do with the color of the water from the spring in the Temple. Dogan was surprised by the color of the water; it was not clear as it had been in the past. This could mean that Jacob's death allowed his rival to enter the source of the spring, under the Temple, contaminating it. It seems pretty clear that Jacob intended to use Sayid as a new body, but that it was important that Sayid not die. It seems significant that the water was dark, and also that Sayid died and then seemed to come back to life. That hasn't happened before, and in conjunction with the apparent victory of Jacob's rival at this time, it points to Sayid being connected to Jacob's rival in some fundamental way.
It all comes down to"why". Other than getting off the island, what purpose would it serve for Jacob's rival to go home? What does he stand to gain if he does escape the island to whatever he considers "home"? And for that matter, what would be the consequence, such that Jacob has been there for some indeterminate time to stop him?
Obviously this only scratches the surface; the momentum gained by the decision to give "Los" a definitive end date continues to drive the series to new creative heights. The writers are already beginning to answer crucial questions and provide perspective, even as those answers give way to new questions themselves. At this point, it comes down to whether or not the critical questions will be addressed. This episode has begun to provide the roadmap.
Overall, this was another strong start to another highly-anticipated season of "Lost". Once again the format has changed, and once again it stands to open up storytelling possibilities that were previously unavailable. The thrust of the final season appears to be established, and now it's just a matter of letting it all unfold.