As I recall, the horcruxes were each destroyed differently. The fire destroyed the diadem, not the snake's tooth, and I don't remember the goblet, but it seems to me they simplified the horcrux part of the book story for the benefit of the film story. And for all of the bigger name stars who were there for brief appearances as a reminder of their roles in the story before, I think they should have defaulted to some of the characters who were more important to this particular story. Also, I remember when Mrs. Weasley killed of Bellatrix, in the book she took on more the demeanor of a warrior, and wasn't just a pissed off mom. But maybe I'm elevating that - that family of characters were shaved way down in this last movie.

I'll have to reread the book.

I saw both the 2D and 3D (because I could - they were shown back to back in different rooms in the same theater) and I can't say that there was enough in the 3D version to cause a preference for the format.