Jack had a kid with Juliet because although they were all still bound together, the influence that Jacob had had on their lives from a very early age was gone, you're familiar with 'the butterfly affect' yes?

A fair bit of the science of the alt dimensions and godly powers inside the earth/history of the civilizations that built the temples (Jacob enchanting the water etc) were left unexplained but the over all purpose of the characters journey was made clear and was resolved.

I could get all geeky because I didn't get some psuedo physics lecture explaining about the forces within the cork etc but for me Lost became more about the Characters... that's what makes any good story and I was satisfied with the closure that they all got.

Locke was vindicated, his faith in fate and purpose was right all along

Ben was almost redeemed and found what he was looking for, in both worlds

Sawyer got 'duh hell off of dis island' ...and of course found hapiness in sideways world

Jack accepted his situation and stepped up to the job also saving his friends and allowing them to escape. Always a leader he was actually able to lead them somewhere

Richard was able to age and let go of his fear of death and punishment

etc etc bla bla bla