I might have the answer to why Jack was supposed to put something of Christians in Locke's coffin.
It has to do with the power of rituals. Like we saw with the drinking of the river to give the new protector of the island his/her power. The same goes for Jack having to go through the ritual of putting Christian's shoes on Locke, it helped him believe they would reach the island again, thus helping flight 316 get to the "window" and their destination. One of the themes of LOST, leap of faith and believing.
On another note, I was happy with the ending in regards to the after life, but am starting to feel a bit conned in regards to so many mysteries not being answered. I know it was about our beloved losties, but they (the writers) really put focus on mythology and sci-fi as well. I can see a few (mysteries)to be left lingering but there a long list, just to name a few:
Christian's body missing
What the heck was Jughead all about
Who was in the cabin
Why Smokie was created from MIB
What side was Eloise on?
Why babies could not be born on the island
Was MIB & Jacob's mom Smokie
the list goes on and on...
Just saying Jacob & MIB were "just some fellas" come on.
With all that said. I loved the journey of LOST and hope that some of the mysteries will be answered in the DVD.