All of the plot changes were made to make the film flow better as a film . They were clearly made because they would have required a.) too much explanation b.) more characters and actors c.) too much time.
The only two unnecessary parts were when Harry and Hermione were dancing and then when they were naked in the Horcrux. The only change in plot that I really wanted to see was Wormtail's death being done correctly. I wish more tension had been built in the particular scenes and that the wizarding war was more prevalent (i.e. Harry walks around too much and there's not enough threat that he is in constant danger).
Other than that .... mischief managed.