I loved the episode because of how much they DIDN'T say. They could have gone on and on and on with references to things we already know, but they chose to make Jacob and MiB's background a story in itself, not just a series of connective threads.
The thing I first considered when I saw that pool with the light coming from it was that that was the spring that the temple would eventually be built above, but obviously it turned out quite different.
Something I'm wondering about is if Jacob actually caused the "darkness" that he talked about to Richard in Ab Aeterno. It seems the island was once a beacon of light, but, by sending his brother in there, he made it a cork for the darkness, a darkness that his twin brother now personifies. I sense some sort of redemption of Jacob and MiB together that will revert the darkness back to light; that will make the island what Locke thought it was in season 1. This isn't a theory...just meanderings
In Ab Aeterno, MiB told Richard that Jacob stole his body. We can see that he appears as himself after his body was "stolen" to Richard and Jacob for quite some time afterwards. It would seem now that his true form is Smokey, and that any body that he appears in, including his own and Christian's, is a sort of costume; a facade. My curiosity lies with in what manner his appropriation of Locke's body is different, why he is "stuck" as him.
The only drawback I have about the episode was the flashforwards to Jack, Kate, and John. That seemed very obvious and corny to me, but I didn't really get hung up on it. I guess it was just a stylistic choice that I didn't appreciate.
That barbarian tribe was SOOOOO cool!
So those are my first-watch reactions...I just put them on here as some food for thought...please comment.