Cho, Marietta, and Hermione's Hex
Book: Marietta Edgecombe told Umbridge of the Room of Requirement, but said nothing more once Hermione's SNEAK hex activated.
Movie: Cho Chang told Umbridge of the Room of Requirement under the influence of veritaserum during Umbridge's questioning of the students (seen in the film, and later mentioned by Snape in Umbridge's office, that she had used his last bottle to "interrogate Miss Chang").
Harry and Cho
Book: Harry and Cho date on Valentine's Day and have a shouting fight about Marietta after the D.A. is discovered. They do not talk to each other afterwards and soon break up.
Movie: Valentine's is cut entirely from the film. After the betrayal, nothing is seen of them together.
Harry's Punishment from Umbridge
Book: Harry is forced to attend several detentions with Umbridge, where she cruelly makes him write the line 'I must not tell lies.' repeatedly with a special quill, which magically cuts into Harry's hand and uses his blood for ink, leaving scars on his hand.
Movie: Only one time is Harry shown attending a detention with Umbridge on his own, where he receives the same punishment. Harry later endures it again, along with Ron, Hermione, and the rest of the members of Dumbledore's Army after Umbridge discovers the group. Later it's revealed that Umbridge uses it against several students for various 'crimes'.
Dobby and the Room of Requirement
Book: Dobby tells Harry of the Room of Requirement, and later comes to the Room to warn Harry Umbridge was coming.
Movie: Dobby doesn't appear in the film at all. Instead Neville accidentally discovers the Room of Requirement, and the D.A. learns they've been discovered when Umbridge blasts the hidden door to the Room, hence all the D.A. members save Cho, serve detention. Also, the Room of Requirement has a bulletin board where the list of members, a picture of Cedric Diggory, and the picture of the original Order (Sirius gave it to Harry in the film) are there.
Mrs. Black, Buckbeak and Phineas Nigellus
Book: Mrs. Black's portrait can occasionally be heard shrieking in Grimmauld Place. Phineas speaks with Harry from his portrait, and later visits Grimmauld Place in search of Sirius, after learning of his death in the Department of Mysteries. Buckbeak is kept in a room upstairs in Grimmauld Place.
Movie: Mrs. Black's portrait was not shown on screen, although it can be heard muttering behind a veil when Harry first meets Kreacher about mudbloods and the defilement of her house. Buckbeak is omitted from the film. Phineas appears only briefly, in his portrait in Dumbledore's office, after the attack upon Mr. Weasley in the Department of Mysteries.
Book: While at King's Cross, the group are escorted by the Advance Guard. Sirius comes along but only in dog form.
Movie: While at King's Cross, Sirius arrives in animagus form changing into human form in a waiting room. Harry enter the room with Mad-Eye Moody and Tonks guarding the door. Sirius gives him the photo of the old Order of the Phoenix. (In the book, Mad-Eye Moody showed it to him.)
Book: Harry, Ron and Hermione meet her in the train to Hogwarts.
Movie: The trio meet her in the carriage to Hogwarts after the train ride and is introduced by Hermione instead of Ginny. Though Luna is reading an edition of the Quibbler, there is no mention made of the fact that her father is the editor.
The Noble and Most Ancient House of Black
Book: Mrs. Weasley and the children are cleaning the Black house and find many dangerous family heirlooms, among them a heavy golden locket which no one can open. Sirius shows Harry the Black Family Tapestry explaining about his brother Regulus and how he became a Death Eater but later defected and was killed. He also revealed to Harry that he is related to the Weasleys, Tonks, Bellatrix Lestrange, and the Malfoys.
Movie: Harry finds the room with the Tapestry in Grimmauld Palace during Christmas Break, and Sirius explains some things about it. Regulus and his other relatives are not mentioned at all, only Bellatrix. The cleaning scene is omitted and the heavy golden locket is not mentioned at all.
Book: The last day of the Christmas holiday at Grimmauld Place, Snape tells Harry and Sirius that Harry will be having Occlumency lessons back at school.
Movie: Immediately after Harry's dream about Arthur being attacked (before Christmas), while everyone is in Dumbledore's office Harry learns he will be having Occlumency lessons in Snape's office, starting immediately.
Book: While Snape is out of his office, Harry looks into Dumbledore's Pensieve, used by Snape. In it, he views glimpses of part of Snape's childhood, the end of an O.W.L. exam, and as the students exit onto the grounds, Harry views his father bullying Snape while the rest of the Marauders look on. Lily Evans makes an appearance and scolds James Potter.
Movie: Harry uses the shield charm on Snape's Legilimens charm and then sees a quick glimpse of Snape's childhood and James Potter performing a spell on Snape, with the Maurauders in the background. Lily is completely cut.
Book: Ron and Ginny join the Quidditch team, and come to some great success. Then after a big fight with Malfoy, Harry, Fred and George are banned from Quidditch for life by Umbridge.
Movie: Quidditch isn't mentioned or seen at all in the film, the first of the films to remove the subject entirely.