Mole: A quiet, ordinary fellow who gets caught up in huge adventures who arguably shares the role of spotlight character with Mr. Toad. Softspoken and a dedicated homebody, he nonetheless sometimes gets wild urges to go out into the wild and "Hang spring cleaning!" Eventually, he adapts to a busier life. He's good with children.
Ratty: The water-rat. Somewhat of an odd friend for Mole, but fiercely loyal to his friends. Says that "There is nothing — absolutely nothing — half so much worth doing as simply messing about in boats," and owns a Cool Boat to punt down The River. Acts as the voice of reason when Badger isn't present, and is somewhat annoyed by the fast pace of modern living.
Mr. Badger: Something of a British Hermit Guru, he lives alone in the middle of the Wild Woods. One would expect him to be huge and terrifying, which he is. But he subverts it by actually being rather nice, sheltering guests and being fond of children. Despite his status as a Gentle Giant, he can — if necessary — be deadly in defense of his friends.
Arson, Murder, and Jaywalking: Toad is convicted for stealing a motor-car, dangerous driving and cheeking the police. Ironically, the Clerk is more lenient with the first two crimes. Although never revealed, Toad's cheek is clearly indicated as "imaginative" and "gross impertinence". Given his flamboyant, conceited attitude, it's not hard to believe.
Ascended Extra: Otter is promoted from "most major of the minor characters" to a full fledged major character in the sequels.
Carnivore Confusion: The narrative says it's against animal etiquette to actually discuss it, but the subject is touched upon by Rat, when he describes the inhabitants in the Wild Wood:
"Weasels — and stoats — and foxes — and so on. They're all right in a way — I'm very good friends with them —pass the time of day when we meet, and all that — but they break out sometimes, there's no denying it, and then —well, you can't really trust them, and that's the fact."
Character Development: Over the novel, Mole comes out of his shell, and Toad settles down to become serious and respectable by the end. Badger also becomes a little bit less reclusive, shown in the epilogue. Grahame pointed out in a later interview that Toad would eventually turn back to his old ways. Mole is the only character whose development would stick.
Characterization Marches On: For the sequels by William Horwood, the main characters have slightly changed personalities. Toad's been hit with some Character Exaggeration and become even more egotistical and ridiculous (though with frequent bursts of extravagant generosity), while Ratty's become a spiritualist of some sort who talks to the river. Only Mole remains more or less the same.
Chekov's Gun: Toad's tongue in the Terry Jones version. At first it's just an amusing Furry Reminder (he uses it to eat a dragonfly), later he uses it to help him escape over a high prison wall.
Cloudcuckoolander: Mr. Toad, at times. "A motorcar! Poop-poop! Poop-poop!" (Or, in some editions, "Beep-beep!")
Cool Boat: It's just a punt, but Ratty's boat is described as beautifully painted and gaily decorated, and there's always a picnic basket on board. Unfortunately, it gets sunk near the end of the novel thanks to the stoats.
Crystal Dragon Jesus: Pan, who is in fact the Savior, but for animals instead of humans. Interestingly enough, the chapter "Dulce Domum" has young field mice singing a Christmas carol that invokes and pays homage to Mary, Joseph and the Christ child. note Animal religious practices of this world must be quite fascinating.
Development Hell: A Darker and Edgier CGI version of the story was supposed to come out in 2012. It's been 2 years and nothing's happened, so it's presumably entered this trope.
A Dog Named Dog: All the principal characters are either this or Species Surname. (Since they only appear to have one name apiece, it's hard to tell which.)
Drives Like Crazy: Mr. Toad's second defining characteristic; he wrecks five cars a week, on average, and has to be locked into his room to try and dissuade him.
Ratty: Once, it was nothing but sailing. Then he tired of that and took to punting. Nothing would please him but to punt all day and every day, and a nice mess he made of it. Last year it was house-boating, and we all had to go and stay with him in his house-boat, and pretend we liked it. He was going to spend the rest of his life in a house-boat. It's all the same, whatever he takes up; he gets tired of it, and starts on something fresh.
Food Porn: The stew Toad dines on, which contains no less than seven animals, is lovingly described. Toad's expression of rapture in the accompanying illustration doesn't help.
Also the contents of Ratty's picnic basket.
"What’s inside it?" asked the Mole, wriggling with curiosity.
Not particularly strong, but in a lot of the artwork, the main cast are much, much bigger than the stoats and weasels. Also, while the main cast wears clothing, Otter wears none.
Also: Toad has a horse called Alfred. While he is an actual quadrupedal horse who carries the caravan, he does seem to be sentient. Strangely, this is one of the few animal characters not named after his species.
Leitmotif: In the TV series, each character, e.g. Mole, Toad, has a certain musical theme.
Lions and Tigers and Humans... Oh, My!: Zig-zagged. Most of the animals live in burrows (albeit in very human-like comfort) and have little or no interaction with humans. Mr. Toad, on the other hand, lives in an actual house, drives cars, is put on trial in a human court, held in a human prison, and escapes by disguising himself as a human washerwoman. During his escape no one suspects that he's Mr. Toad until he actually announces it when he rides off with a barge woman's horse. And he also interacts on a more-or-less equal basis with all the other animals.
Loveable Rogue: Toad is considered an epitome of this. Although concieted, reckless and even kleptomaniacal at one point, he genuinely cares for his friends and shows great humility and distress upon learning of the hardships they suffer on his account.
What Measure Is a Non-Cute?: The weasels, and the related ferrets and stoats, are all nasty little buggers, sneaking into Toad Hall to take it over while Toad is out. They're eventually let go with a warning, though, as they promise to be good after being thrashed by Badger.
Wholesome Crossdresser: In a humorous incident, Toad escapes prison disguised as a washerwoman with clothes from the jailer's daughter, and manages to wind up disguised on a train outrunning the police.
Wolverine Publicity: Several adaptations aren't named "The Wind in the Willows" but rather "Toad of Toad Hall", "The Adventures of Mr. Toad", etc. etc.