The Island mysteries have always been secondary.
Secondary, but not non-existent, and certainly not a distant second. You can't have characters without plot and plot without characters. It's a symbiotic relationship.
And even as character stories, I guess I'm kind of missing a lot about why their afterlife was really important at all.
I think it answered most of the important things, like why they were there (Jacob needed a replacement) and stuff like that.
Maybe I see this differently, but if I was any of the four who Jacob explained this to, I would've asked, "But why me? Why me when there are so many in the world who are equally, if not more troubled? What exactly did you do to your brother? Why was everything so predestined to lead to this?" On top of that, I think Jack should've asked Jacob a billion more questions, but instead takes a leap of blind faith. Now, I think that faith can sometimes be good, but the transition of his character from being skeptical to just automatically accepting fate and faith without question to be somewhat of a stretch. Maybe I'm expecting more out of these characters.