There are certain scenes in the sixth book that define it for me - Malfoy's breakdown in the bathroom that was inexplicably turned into a pissing contest in the movie; Snape's "hatred and revulsion" etched onto his face as he kills Dumbledore, depicted in the movie as..apathy?; the flight from the castle - did they really just walk out the front door with no pursuit except Harry? - and the Snape/Harry showdown that was all too anticlimactic.
Plenty of scenes were invented to, I suppose, speed up the narrative. Jim Broadbent as Slughorn was far and away the most interesting character, and not just because he was new. The character was just well done. The principals...well, I'm not sure that Daniel Radcliffe has quite mastered the whole crying on camera bit yet. Maggie Smith looks ancient. I'm glad that the second incarnation of Dumbledore is dead, because Michael Gambon never did it for me.
"Worst. Movie. Ever. I will only see it three more times...today."