I just watched Harry Potter and the Order of the Phoenix.
Spoilers for book/movie 5 follow
To recap the plot, the Ministry of Magic is trying to suppress any news that Voldemort is back, they are interfering with Hogwarts in the form of Dolores Umbridge, the new teacher of defence against the dark arts, and Voldemort is trying to do something nefarious yet clandestine.
This fifth instalment in the Harry Potter movie series was a faithful adaptation of the book but it failed to convey the emotions which form the basis of the fifth book's themes. The fifth book is probably the most frustrating book because Harry is an angry teenager, who feels that the adults are still treating him like a child, he doesn't know how to deal with girls, and also Dolores Umbridge is making Hogwarts living hell for the students. Of these themes, only Dolores Umbridge is properly portrayed by the movie; the other themes merely get their token scenes to placate the readers. For example, a significant portion of this book deals with Harry's relationship with girls, Cho Chang specifically. In the movie this is represented by a total of 4 scenes. More importantly, not enough time was spent dealing with Harry's relationship to Dumbledore and the rest of the Order of the Phoenix. Throughout the book Harry is constantly sidelined while the Order tells him to stay out of trouble, etc, and Dumbledore is absent or ignoring Harry. This isolation has significant consequences for Harry, but the movie doesn't explain this to the same extent as the book does. It's only because I read the book that I realized that Dumbledore was more absent than normal. As with Cho Chang, the Dumbledore storyline is shown with a scant handful of scenes that do not do justice to the story.
Granted: this book is one of the longest yet and thus is more challenging to filmmakers. Unlike the Goblet of Fire (which, I might add, also made a rushed and confusing movie) there is hardly any action, thus there is lots of dialogue and exposition that needs to be crammed into a few hours. The movie thus crops every sub-plot until it's recognizable only to those who've read the story. Anyone who hasn't seen the other movies or read the books won't know why that red-headed girl is following Harry around at the end (it's Ginny Weasley), nor who the man with the long white hair is that leads the Death Eaters (it's Lucius Malfoy). Now, these details aren't critical, but basically if you don't know the story already you'll be left to dangle along, unsure about what's happening and bored by the lack of excitement.