Most egregious fact missing:
It is not told that Harry owns the house on Grimmauld Place and thus is Kreacher's Master. So, when he's spilling his guts to Harry about the locket, he could have, in theory, told that to anybody if he is not Harry's house elf. For sure, he told Dumbledore at some point in the time of Book 6 (he was working in the kitchens with Dobby).
But, this was taken out of Movie 6 in favor of the "flirting with the waitress" scene. Frustrating.
His betrayal of Sirius Black in Book 5 was cut out in Movie 5. As I wrote in another thread, elf magic might be too confusing to keep in a movie. It has to be explained, and reading about it seems easier than talking on a screen about it.
Oh, and Kreacher could have done all those things in Malfoy Manor. Harry forgot that, to, in Book and Movie.