The Kate/Aaron stuff made no sense in the context of Kate's scene with Jack in "316". If it was as simple as Kate deciding that she had to leave Aaron with Claire's mother, while she goes to search for Claire, why didn't she just tell Jack that? Why doesn't she tell everyone that? Why say "Don't ever ask me about Aaron again", which surely must freak Jack out? Doesn't Jack deserve to know that Aaron is OK?
I think the writers are too in love with the idea of turning everything into a mystery, even when doing so is uncalled for, and defies all logic.