Alternative Character Interpretation: Everyone (except Jack). Some people include their backstories from the book in the movie, while others prefer to keep them divided. Even the writer and some artists working in Dreamworks have their own way of viewing the series.
Pitch. Was he only manipulating the other characters (mostly Jack), and wanted to create a world filled with fear for his own selfish pleasure? Or was he a very lonely man who only wanted to be believed in, and truly wanted Jack as his friend? The scene in Antarctica shows a more complex side of him, making it hard to tell which interpretation is right. Of course, you could Take a Third Option and say they're both right.
According to the DVD's audio commentary and the movie's script, Pitch was definitely genuine in his opening up to Jack, and truly hated being alone. Pitch probably empathizes with Jack, and may really view him as a potential friend.
Bunnymund. Some people think he's the same as in the book, while others tend to keep them divided. The fact the two versions are vastly different doesn't help.
Tooth and Jack's relationship. Some people see her as a mother figure for Jack. Others are okay with her being a possible love interest.
Angst? What Angst?: When Jack discovers his memories of his previous life, he chooses to focus on the fact that he saved his younger sister, instead of the fact that he died in the process.
Broken Base: Tooth punching Pitch in the face near the end. A justifiably angry Mama Bear, or a step too far?
Critical Dissonance/Acclaimed Flop: The film received mostly positive reviews from critics, but cost $145 million to produce and only made $103.2 million at the North American box office, becoming Dreamworks Animation's lowest grossing film domestically since Flushed Away. According to Wikipedia, the total worldwide take was $303 million, but Dreamworks saw little of that money once budget costs and the distribution fee owed to Paramount were tallied up — they ultimately lost $83 million on it.
Die for Our Ship: Some people love the Frostbite/Rainbow Snowcone (Jack x Toothiana) pairing. Others see Tooth as a mother figure for Jack. Some people support crossover pairings, like Jack/Elsa or Jack/Rapunzel. Others prefer Jack/Pitch or Jack/Bunny.
Draco in Leather Pants: Pitch has as many fangirls as Jack. Some have even said, without a single trace of sarcasm, that he's just a complete woobie who should be forgiven for all his misdeeds (which, y'know, includes his attempts to rule all the children with fear). More fangirls have gone on to compare him to Ralph from Wreck-It Ralph in that they're villains who want to be accepted, even though they have almost nothing in common and that unlike Pitch, the most villainous thing Ralph ever did was smash a cake.
Despite only a few seconds of screentime, the little mouse is beloved in Latin-America. The Latin-American dub has the Tooth Fairy say that the mouse is from the "Latina division", because in Latin-America, it's a little mouse named Perez who leaves children money when they lose a tooth.
The fans are also very fond of Jack's sister (who isn't named and only appears once) and Pitch's daughter (who doesn't even appear in the movie).
Estrogen Brigade: Jack Frost. Lampshaded rather nicely in a comic drawn by one of the film's story artists.
Fan Nickname: Heavy Weapons Santa for North, especially among the Team Fortress 2 fandom, due to his resemblance in size, mannerism, and nationality to a certain, well, Heavy Weapons Guy.
Fanon: Jack's sister is never given a name, but the most popular name in the fandom is Emma.
Fan-Preferred Couple: The three most popular ships are Jack/Tooth, Jack/Bunny and Jack/Pitch. Jack/Jamie, Bunny/Tooth and Pitch/Sandy aren't as popular but still have a following.
Genius Bonus: The magic sleigh, heavy furs, reindeer and going up and down chimneys are all straight from Siberia, where Saint Nicholas was interpreted as a kind of super-shaman, who flew out the smoke-hole of his medicine tent and travelled the spirit world in a sleigh pulled by reindeer. Now consider that Santa's accent isn't just Russian, but eastern Russian.
Verypredictably, the Jack/Elsa crossover ship has quickly grown to rival the popularity of the other three, and some fans pair him with Elsa's sister Anna instead. There's also a small but vocal minority who speculate that Elsa and Anna are somehowJack and Rapunzel'sdaughters.
Lucky Translation: The Italian equivalent of "sweet dreams" is "sogni d'oro", which literally means "golden dreams".
A lot of the fanart is made to draw the Guardians as humans. Tooth is undoubtedly the most popular for this, with Bunnymund following.
Early on in the fandom, someone pointed out that without audio for Jack's "and the Easter Kangaroo" line, it looked like he was saying "You're an asshole". Double points for him pointing towards the camera. Needless to say, it caught wind with the other fans.
Ship-to-Ship Combat: A rather odd crossover example, but fans who ship Jack with Rapunzel sometimes get into dispute against those who prefer to ship him with Elsa, while the latter group of shippers are also frequently at war with fans of Hans/Elsa.
The Woobie: Jack Frost. He never sees his little sister again after saving her, then spends 300 years all alone with nobody knowing he exists.
Bunny becomes this after Pitch's nightmares destroy his eggs and then cause the children to lose their belief in him.
Pitch too (albeit of the Jerkass variety), once you know he came through the same hell as Jack but wasn't as lucky as he was. Then when you add in his backstory from the books...
Tooth. In the film, children stop believing in her, making her palace disintegrate and causing her wings to stop working. Not to mention her fairy helpers (e.g. Baby Tooth) are put in danger and if you factor in her backstory from the books...