The biggest problem is not only was Hermione made into this "Super Hermione" but Ron was made to look like a bitch in POA. All of Ron's good lines were given to Hermione (i.e. "If you want to kill Harry you'll have to kill us first", which Ron said while standing on a broken freakin leg!) and Ron acted all timid around Malfoy. In the book they constantly have to hold Ron back from killing Malfoy, but in the movie Ron was all "shut up Malfoy" before practically hiding behind "Super Hermione".
It all stems from the fact that Steve Kloves (the screenwriter) has this hard-on for the Hermione character.
Another thing is in the book POA Hermione starts to crack under the pressure because she's taking so many classes using the Time Turner. That's one of the main reasons she hits Malfoy. In the movie she's as calm as ever. Her cracking makes more sense to her character since she's so focused on school and grades (evidenced by her boggart during the DADA Final in the book)