[quote=@Majoras End]
<Snipped quote by Fabricant451>

Really? I did play through vanilla KH3, so there's not much to miss?
[/quote]

It's mostly cutscenes that are front loaded, then it's just the Keyblade Graveyard part again with some added scenes to basically elaborate on some stuff that happened during it. 

Then there's the second part that is basically Nomura giving a middle finger to everyone who thought the game was too easy. You're essentially paying for a directors cut of the finale of the game and then 13 superbosses.