Personally, I love this movie because it has a colorful, developed cast of characters.
Basil is not your average Disney hero, having no love interest at any point in the movie; he is also a super-intelligent character and often bursts into fits of scientific observation. His facial expressions are timed well and are very well done, as well as his general movement. He is very excitable and you can tell he adores working on cases. He doesn't really like children, as you can see from the way he treats Olivia in the beginning. You really get a feel for Basil when watching him, and he's not easy to forget. Kids may not understand half of the things he's saying, but it's fun to watch.
Olivia is the little heroine mouse who starts Basil on his case, losing her father to a mysterious peg-legged bat of whom Basil is aware of. She is extremely plain-drawn and has no emphasis on her gender really, save for the bow in her hair and her voice, making her very realistic. She also acts alot like a realistic little girl, in her laugh, in the way she reacts to things, even in the way she exclaims "father!" when reunited with her toymaker father. Her curiosity is classic, and she has a high adoration for Basil, though he doesn't appreciate her at the start of the story.
Dawson is the elder mouse who finds Olivia looking for Basil of Baker Street. He isn't sure of who Basil is, but they end up finding him together. Dawson is, of course, Watson in mouse form, and helps Basil in his first case, being this movie.
Ratigan is the villain played by the late horror film star Vincent Price, and quite well in my opinion. He has the likeness of Mr. Price in the way he moves and acts. Of course, nothing is weirder than hearing him sing, but it happens twice in the movie-- "The World's Greatest Criminal Mind" and "Goodbye, So Soon" were both written specifically for Price. He is not one of the more frightening villains as far as his looks, and is unique in that up until the climax he uses his intelligence to battle Basil, instead of the strength he obviously posesses. On a special note, Ratigan dislikes being called a rat for some reason, though I cannot remember why.
Fidget is probably one of the darkest villain henchmen in a Disney movie. He is frightening to children in appearance and voice, and the music accompanying him adds a comical aura. Fidget is by far my favorite Disney henchman, just because he's funny. The one thing that bothers me, though, is that a few times in the movie, his peg leg switches from right to left. I think it was just for the reuniting scene in which Olivia steps on his only foot, so it was switched.
The movie has many fast-paced scenes, including "Investigation At The Toy Shop" and "Big Ben Chase", among others. My personal favorite is "Big Ben Chase," because it is one of the longest Disney climaxes and it is a very unlikely setting. The movie itself is about Olivia's toymaker father being kidnapped by Fidget, and her turning to Basil for help. Basil's nemesis, Ratigan, is linked to the bat, and Basil, who was reluctant and annoyed by Olivia's protest at first, jumps at the oppertunity to see if he can finally defeat Ratigan. The movie goes from there with its "many twists and turns", to quote Basil.
If you want a fun-filled, unpredictable adventure, The Great Mouse Detective is your best bet. Especially if you're a Sherlockian or Eve Titus fan who has yet to see this exiciting portrayal of Basil of Baker Street.