This is something I haven't seen many ppl talk about yet but I believe is very important. Ab Aeterno actually made me care about the Flash-Sideways. Before this episode, I couldnt figure out how this side timeline would be in anyway important to the main one. But this episode sums it up very well.
Essentially, if Jacob is telling the truth when he tells Richard that the Island is the "cork," that keeps evil out of the world, then in the flash-sideways, evil has been spreading in the world, since at least the '60s, or whatever time of year the Incident happened. And if evil has been spreading all this time, then all the little differences we've seen so far, all the things that seem to be 'better' for our characters, are actually 'worse.'
If Jacob was telling the truth, which I by the way, believe he was, then Jack having a son is actually worse for the world, than his not having one. Sawyer being a cop does more harm than it does good. Ben being a history teacher rather than a border-line psychopath (same thing?) is actually not the best thing for the human condition.
And I never could've seen that idea coming in to play with a plot device such as this. I think it's been obvious since day one that the Island was more than just some uncharted territory off the coast of Fiji. So the idea that it alone protects mankind from "darkness, malevolence, evil," isn't too far off from general expectations. In a typical sci-fi work such as Lost, the writer(s) would only describe what the world would be like without it's almighty protection, but they would never actually take the time to show you that world, and have you get vested in the character's in it.
So not only did Lost break ground by taking us to this "dark" world, they made it seem like not such a bad place. By making the flash-sideways world pretty banal, normal, and calm, they've made it very similar to the real world - a world where your biggest struggle is to connect with your son, or to make peace with your past, or to deal with your unfortunate circumstances.
And so, in light of this development, we're forced to ask the question: is it possible that all the f*cked up things our Losties went through in the original timeline, was actually good for them, and good for the world? Even more, what if in our own lives, we knew for a fact that somewhere out there, was a magical island that protected us all from evil, would it make it easier for us to swallow the things that dont make sense? When our father died, or our wife left us, or our children wanted nothing to do with us, would it make it all better if we could sit back and say, "Well, must be for the best. Must be by the will of Jacob."
It's something we religious folk do all the time, or at least try to, which is rely on our faith that things will work out for the best, even when it's hard to understand how or why that is. And, for those of us not so religious, it's a question I hope Lost is challenging you to ponder. Not about faith, but about purpose. Yours, and mine.