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Voldemort, especially, is wronged here. Dumbledore completely glosses over the significance of the items Voldemort has chosen as his horcruxes. Just by the movie, you're left with the impression that Voldemort may have chosen a soup can to hold a piece of his immortal soul. In fact, his vanity and self-aggrandizement drive him to choose items of immense value and significance. Without this little bit of information, the quest left to Harry et al is impossible. Furthermore, Voldemort's character is done wrong by the omission of the key dialog from the book between Harry and Dumbledore in the cave. The relationship between Ginny and Harry also gets the shaft, which is harmful to exposition of Harry's motives in the rest (and also relevant to the dreadful epilogue from the seventh book. Here's to hoping they axe that in the eighth movie. Probably the most contrived bit of writing I've ever seen published, apart from Ayn Rand.). There was some other stuff that I think could've been better, but I think a lot of that is countered by the fantastic awkward atmosphere developed on the first half of the movie.
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For what it's worth, the scene where Harry and Dumbledore are being "attacked by an army of Smeagols" is straight out of the book. What is missing from the movie is the information about how Voldemort uses what are essentially zombies as an army and they are in the lake to protect his horocrux. When the lake water is disturbed, they rise to capture who ever might have been attempting to steal the trinket. These "zombies", or inferi as they are called, are mentioned several times early in the book.
Also from the book, Draco is appointed the task of killing Dumbledore as punishment for his father having failed to retrieve the prophecy in the last story. It is implied that Voldermort fully expects Draco to die while trying to accomplish the task.