One of the chief examples: the Slytherins in the battle of Hogwarts. The fact that none of the Slytherin students and of the teachers only one (Horace Slughorn) battles Voldemort & Co. along the other houses in the final battle has come in for (justified, imo, given the "we must unite" theme earlier) criticism. (Sidenote: doesn't mean I think the majority of Slytherin students should have stayed, that would have been unrealistic given what was previously established, but a couple of them? Absolutely.) The movie manages to make a questionable creator choice worse and manages to rob it of some of the good aspects. Compare and contrast:
Film: Voldemort telepathically tells everyone at Hogwarts either to surrender Harry to him, or be killed. Pansy Parkinson suggests to grab Harry and hand him over. Students of the other houses rise to place themselves protectively around Harry. McGonnegall gives the order to escort all the Slytherin students into the dungeons. Cheers from everyone else.
Book: Post-Snape's flight, McGonnegall tells a flustered Slughorn who asks what the hell just happened that Slytherin House will have to make a choice as to where their loyalty lies and that they should either stay and fight, or leave. Slughorn is somewhat panicked at the thought of going up against Voldemort. Voldemort does his telepathic "Surrender Potter, or else" demand thing. Pansy suggets handing over Harry. Non-Slytherin students surround Harry. McGonnegall orders Pansy and the rest of the Slytherin students to be escorted off the Hogwarts premises into Hogsmeade (cheers from the other houses); later adds the younger students of the other houses and anyone of the older ones who doesn't want to join the fight to the general evacuation. All the Slytherin students, the younger kids and some, but not many of the older ones leave. Slughorn supervises their evacuation from Hogwarts, then returns along some of the Hogsmeade inhabitants in time for Voldemort's second wave of attack on the castle. (Slughorn is still, as Harry notices when he passes him, in his emerald pyjamas.) There's no suggestion Slughorn shirks fighting; later Harry spots him battling Voldemort himself at McGonnegall's side. Now, in addition to the fact that escorting ALL the younger students off the premises of an impending brutal battlefield is a really good (and responsible) idea and that they should have kept it in the film version, at least in dialogue reference if they didn't want to show any evacuation scenes, here McGonnegall's "fight with us or leave" may be unkind but in a situation of impending batlte makes strategic sense (she can't afford anyone staying who is more willing to sabotage than fight), and it comes as a choice, not a preempting condemnation. Also, Aberforth Dumbledore, through whose pub all the students were evacuated, furiously asks Harry why they didn't keep some of the kids of the Death Eaters hostage. Harry replies that a) Voldemort wouldn't care (example to go by: Draco Malfoy, the reader might add), and b) not cool. Which I thought was a great case in point for the type of pragmatic compassion Harry has reached at this point of the saga. It also shows that Aberforth, while a good postumous counterpoint to Albus and delivering criticism of same that the reader is supposed to take seriusly, is no saint, either.
Speaking of Dumbledore criticism, the movies make the baffling choice of keeping some hints of his backstory but never bothering to deliver an explanation, which I think is the worst of both worlds. Either cut out the backstory altogether, or keep the entire Dumbledore backstory reveal. But just hinting there was something up with his family and then later via Aberforth that it has to do with their sister, without explaining exactly what, keeping the photos of the youthful Grindlewald and Dumbledore without any of their connection ever mentioned verbally, and cutting anything in the King's Cross conversation between Harry and Dumbledore that relates to Dumbledore's past and his deep regrets, his self loathing, only leaving Dumbledore in his usual wise mentor delivers exposition role...? Bah.
On another note: something that movies and (a lot of) fanfiction do have in common is that they remove not only Harry's tendency towards sarcasm but also Snape's pettiness. Seriously: Snape is still one of the most interesting and compelling characters of the saga to me, but he's the uncontested winner for "most petty" by far. Fanon usually goes with "but he had to maintain his cover!", but I very much doubt Voldemort's belief in Snape's loyalties depended on such gems as Snape making Harry copy detention notes on his father and Sirius in the year after Sirius' death. Fanon!Snape never does that kind of stuff. (Movie Snape didn't, either.)
Speaking of fanon, I've come across several stories in which Slughorn treats young Severus unfairly and ignores him while favouring Lily. Where the hell does this come from? What book canon gives us: Slughorn was into students who were either talented or socially well connected. Lily was one of his favourites (btw, The Half Blood Prince marks a switch from people telling Harry about his father to people, in this case Slughorn, telling him about his mother), but there is absolutely no indication he didn't also spot and encourage young Severus the potion genius. On the contrary. In the present, Slughorn immediately spots how gifted Hermione is and encourages her, never mind she's a Gryffindor. (As opposed to, you know, Severus Snape, whose behaviour towards Hermione varied between ignoring her, telling her she was an insufferable know-it-all in front of the class and insulting her physical appearance - "I can see no difference".) And then we get this scene, where Slughorn raves about Harry's gift with potions at one of his gatherings (due to, unknown to Slughorn, Harry having "The Half Blood Prince"'s text book):
"Instinctive, you know - like his mother! I've only ever taught a few with this kind of ability, I can tell you that, Sybill - why, even Severus -'
And to Harry's horror, Slughorn threw out an arm and seemed to scop Snape out of thin air towards them.
'Stop skulking and come and join us, Severus!' hiccoughed Slughorn happily. 'I was just talking about Harry's exeptional potion-making! Some credit must go to you, of course, you taught him for five years!"
Trapped, with Slughorn's arm around his shoulders, Snape looked down his hooked nose at Harry, his black eyes narrowed. 'Funny, I never had the impression that I managed to teach Potter anything at all.'
Leaving aside this is JKR at her comic best (and also foreshadowing, given the identity of "the Prince"), this certainly doesn't give me the impression Slughorn used to overlook Snape in Snape's student days, on the contrary. I bet he told him "stop skulking and come and join us, Severus" a lot. :) Consequently, I suspect the idea of Slughorn ignoring young Snape hails more from the subconscious or conscious fanon that child and teenage Severus didn't get any encouragement from not-evil people and therefore had no choice but to fall in with the Death Eaters.
Lastly, Horace Slughorn: non-evil Slytherin who never was a killer and doesn't die is generally an interesting creation. He's not without bias; his initial reactions to Muggleborns doing well is still a surprise, even if the second is to take to the gifted person in question, which makes him an hitherto unprecedented in between as far as the HP saga's fantasy racism depiction is concerned. (I.e. pre-Slughorn, you had on the one hand racists using the term "Mudblood", marked for future Voldemort service, and otoh non-racists utterly believing in equality). And of course, you can argue whether networking and collecting gifted and well connected students is what a teacher should do. But he deeply abhors murder (when we finally get the real memory of him and Tom Riddle, that's what shocks him out of Riddle's conversation - "Seven! Isn't it bad enough to think of killing one person? And in any case... bad enough to divide the soul... but to rip it into seven pieces..."), and while he loves his creature comforts and has an instinct to evade rather than confront past mistakes, he comes through and puts his life on the line when everything is at stake. And as opposed to McGonnegall and Snape, both of whom we see favour their own houses when teaching (while Snape does it more blatantly, McGonnegall is anything but free of this behaviour), Slughorn really seems to reward students according to their efforts; we see him repeatedly giving points to Gryffindor for academic brilliance whereas I'm struggling to recall an instant of McGonnegall giving points to Slytherin for the same reason.