To me it answered
1) how Jacob doesn't age (he's the protector)
2) how Man In Black doesn't age (he's already dead&the smoke monster)
I also hadn't figured out earlier that they are brothers.
I love that they finally explained who Adam and Eve were.
I kinda hate how mommy's boyish Jacob was though. I always thought Jacob was so cool but knowing what his mother was like, and the only person he grew up with D: idk
So Jacob had those kids kidnapped when the plane crashed because he was looking for someone to replace him. But that didn't work out. Every time all the candidates die/whatever, Jacob invites more people. But now that Jacob is dead, he can't invite any more. So this is MIB's last effort to kill every possible replacement before they actually become the replacement, so that he'll be 'free'. Nice...