Adaptation Displacement: Book? What is this book you speak of? The film is much more well known than the source novel. Even the poorly received sequel seems to be more famous.
This is not helped by the ebook being available only on the UK iTunes Store.
The book is far more popular among elementary school students in the US.
All Animation Is Disney: Like most of Don Bluth's work, this mistake has been made, but those who make it are generally a minority. If anything, it's most famous for being one of the best non-Disney animated features ever.
Bellisario's Maxim: Don Bluth's handwave regarding the addition of magic and the amulet to the film without explanation; he simply felt it was unnecessary to explain the Amulet's origins and that its purpose in the film was more important to focus on.
"With regard to the amulet, it is a metaphor for believing in one's self. Remember the quote, "Courage of the heart is very rare, the stone has a power when it's there." It helps symbolize her courage and the power of the stone to help rescue her children...a miracle, if you will. God stuff. Granted, it isn't in the original novel, but we felt that it was much more powerful. Nicodemus says it was Jonathan's, but really just to get her to accept it. We didn't really think it was necessary to explain it further. Seems like we would eat up too much screen time to tell the history of the amulet, when the story was about an innocent widow mouse, who, thru her journey would find out that she has the courage to rescue her own family. Regarding magic, we really believe that animation calls for some magic, to give it a special "fantastic" quality. The stone or amulet is just a method of letting the audience know that Mrs. Brisby has found 'Courage of the Heart'. Magic? Maybe. Spiritual? Yes."
Complete Monster: Jenner is a nasty, murderous piece of work. In contrast to his noble brethren amongst the rats of NIMH, Jenner desires nothing but power. When The Hero Mrs. Brisby comes to the rats for help moving her family and house to be safe from a farmer's plow, Jenner sabotages the moving so the wise and kind leader of the rats, Nicodemus is crushed to death. When he sees Nicodemus bequeathed a special stone to Mrs. Brisby, Jenner attempts to murder her for it. What makes it even worse is Jenner, like all the rats, owes his very life to Mrs. Brisby's deceased husband and the father of her kids and displays zero remorse or gratitude to Jonathan's memory. After his henchman Sullivan finally has enough with Jenner's lunacy, Jenner slashes his throat and attempts to kill his rival Justin, declaring his only philosophy in life: "Take what you can when you can."
Fan-Preferred Couple: Justin and Mrs. Brisby. There's obviously some attraction going on there, but their species difference and the fact that Mrs. Brisby is very recently widowed makes this something of a hot-button subject.
Here's what the actual script has to say about their introduction:
[Justin] notices [Mrs. Brisby]. She has already noticed quite a few things about him.
Genius Bonus: Robert Burns' famous poem, To a Mouse, was inspired by his accidentally destroying a mother mouse's nest while plowing his field (notice the combine is consistently, anachronistically referred to as "the plow" throughout the movie). In the opening line, he refers to the mouse as (translated) a "wee, well-groomed, cowering, timid beast". Remind you of anyone?
Harsher in Hindsight: In one scene Mrs. Brisby jumps out of a bird cage with a string to break her fall. In real life, her voice actress Elizabeth Hartman committed suicide a few years later by jumping to her death.
A mild case, but at some point Mr. Ages asks Justin to take Mrs. Brisby to the library (where she can wait before meeting Nicodemus). Yet it's later revealed that she only has very basic reading skills, and thus would probably not be able to enjoy the reading of a book. In the original novel, when Mrs. Frisby is taken to the library, she meets a younger rat, and the two have a conversation before she gets called to see Nicodemus.
Hype Backlash: An example of an otherwise underground work being pushed on the general public by its small audience. A great deal of big name fans, most notably The Nostalgia Critic, have frequently gone on about how it's one of the greatest animated films ever. The result was that some newcomers were significantly disappointed when they felt it didn't meet the incredibly high expectations its fans had given them.
Magnificent Bastard: Jenner is regarded as one of the most successful villains in film history. Since he makes the falling of Mrs. Brisby's house and Nicodemus' death look like an accident, no one suspects his cruel deed... until Justin finds out about his plans from Sullivan. Had he counted on that as well as Mrs. Brisby warning the other rats about NIMH coming to exterminate them, he would've gotten away with it.
Moral Event Horizon: Jenner crosses this after cutting the rope and causing the brick to crush Nicodemus.
Ship-to-Ship Combat: Back in the early days of the NIMH fandom, there was a good amount of battle between Justin x Brisby and Jonathan x Brisby (substitute Brisby for the Fan Nickname if desired); this was fueled by a still image of Justin and Mrs. Brisby kissing, rumored to be a shot of a scene cut from the movie; Word of God killed this notion fairly quickly, but the fandom picks at it from time to time.
Special Effects Failure: The bulk of the film has impressive animation and special effects work, but some of the scenes of the Tractor and its plow during the Moving Day sequence are obvious live action footage tinted in brown, which sticks out like a sore thumb compared to the rest of the films art. Even Don Bluth admitted he was dissatisfied with the effect in his book "The Art of Storyboard".
Visual Effects of Awesome: Between the lightning sparking off Brutus's halberd, the lightshows from Nicodemus' Crystal Ball, the Mind Screw-y visuals during the NIMH flashback sequences, and the amulet coming to life at the end, the light effects are utterly dazzling.
Done by a staff of three effects animators, no less!
What Do You Mean, It's for Kids?: Subverted. It's for families, and not necessarily ones with very young children. Bluth wanted it to appeal to a broad audience, which explains a lot of the scary imagery and the one casual "damn!" thrown in. This was during a brief pocket in time before PG-13 when you could get away with more family-unfriendly content in family movies and before every animated movie had as PG rating that was really just a "hard" G.