THEY-CUT-OUT-THE-QUIDDITCH-CUP!
I will never, ever forgive them for that.
They've cut out Quidditch completely.
They have also screwed up every movie except for 1&2. They got rid of Quidditch, made Ron and Hermione not prefects, made Harry seem like a madman, and just about wiped out the prophecy. And, Ginny said exactly 4 words throughout the whole movie, 2 of them were spells, and they were the SAME SPELL! And, they've gotten rid of Dobby and given all his jobs to NEVILLE! Neville is NOT a house-elf!
I could go on forever.