Everyone. Some people think the characters are the same as in the books the movie is based on, while others prefer to keep them divided. Even the writer and some artists working in DreamWorks have different opinions about this.
Tooth and Jack's relationship. Some people see her as a mother figure for Jack. Others view her as a possible love interest.
Is he only manipulating Jack in the Antarctica scene, or is his compassion genuine, and Pitch truly wants him as a friend? According to the DVD's audio commentary and the movie's script, Pitch is definitely genuine in his opening up to Jack, and truly Hates Being Alone but in the movie itself, it is hard to tell which interpretation is right.
Angst? What Angst?: When Jack gets back the memories of his human life, he is absolutely delighted: he had a family once and even saved his younger sister! The facts that he died saving her and that, by now, all of the said family is long gone isn't shown to bother him at all.
Colbert Bump: Rise of the Guardians bombed in theaters but still gained its dedicated fanbase. Then, many fans of the very successful Frozen (2013) noticed this movie as Jack/Elsa Crossover Ship grew popular. However, this also caused a backlash agains the movie.
Critical Dissonance: 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.
After the release of Frozen, Jack is very often shipped with Elsa, not just because they're both ice people protective of their younger sisters, but also because Jack's playfulness contrasts interestingly with Elsa's seriousness. The ship's name is "Jelsa" though some people prefer them as siblings and call them "Frost Family". The ship has inspired many fanworks to the point of attracting backlash from those irritated by its popularity.
A small portion of viewers ships Elsa with Pitch instead, because his life's work is fear and her whole problem in the first Frozen is letting fear control her, and/or because both are viewed as loners wanting someone to accept them and their strange power.
There's also a small following for Wendy Darling/Jack Frost, or "Frosting," as Jack's personality is similar to Peter's.
Cult Classic: It unfortunately bombed in theaters and isn't quite a household name like Shrek or Kung Fu Panda, but it quickly developed a sizable, dedicated internet fanbase that persists to this day.
Die for Our Ship: Shipping Jack with Elsa from Frozen (2013) is an odd case of a non-canon Crossover Ship inspiring this treatment. The pairing grew very popular, frequently featured everywhere from fanfiction to cosplay, so some perceive Jack as getting in the way of whatever other Elsa ship they prefer. To let her hook up with someone else, quite a few fanworks humiliate, demonize, or outright kill Jack.
Draco in Leather Pants: Pitch has as many fangirls as Jack. Some even say, without a trace of sarcasm, that he's a complete woobie who should be forgiven for all his misdeeds (which includes his attempts to murder the Guardians and a child) or has done nothing wrong in the first place. This adoration can be attributed to the books of which the film is loosely based on, where he was once a human and had a daughter who he lost making him more tragic. Or, the adoration can be attributed simply to his good looks.
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 very fond of Jack's sister who only appears once and isn't even named.
Escapist Character: Jack is a boy who dies young, becomes immortal, is free to travel the world without responsibilities, and has magic powers while dressed in a hoodie. No wonder he's popular with teenagers and fan artists.
Estrogen Brigade: Jack Frost. Lampshaded rather nicely in a comic drawn by one of the film's story artists.
Evil Is Sexy: Pitch Black. The creators admitted in the official DVD commentary that they interpreted fear as something that "seduces you," so they made Pitch sensual and seductive on purpose. One director even complained that his wife seemed to like Pitch more than him!
Frozen fans and Rise of the Guardians fans often accuse Disney or DreamWorks of stealing the idea of a movie focusing on An Ice Person. Neither invented the concept: in fact, each studio have been very open about both Elsa and Jack Frost being based on pre-existing characters that predate both films, Elsa being inspired by a nineteenth-century tale written by Hans Christian Andersen and Jack Frost originating in folklore. Disney had been working on a film based on the Snow Queen on and off for decades, since the days of Walt himself, and leaked pre-production art was already online by 2008, the year Dreamworks purchased the film rights to William Joyce's The Guardians of Childhood book series, which united the variety of folkloric characters seen in the film, including not just Jack Frost, but other well-known childhood staples such as the Sandman and the Easter Bunny.
There is small scale rivalry with the The Loud House fandom, particularly fans of Lincoln Loud due to their physical similarities, as well as both being Launcher of a Thousand Ships within their fandom. Any complaints of Lincoln's ships often end up with fans complaining about the fans' Sacred Cow treatment of Jack and his many crossover ships.
Fanon: Jack's sister is never given a name, but the most popular name in the fandom is Emma.
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 traveled the spirit world in a sleigh pulled by reindeer. Now consider that Santa's accent isn't just Russian, but eastern Russian.
Hype Backlash: The massive popularity of the Jack/ElsaCrossover Ship and the heated arguments for and against it have gained a backlash not only against the movie but also its fans and even William Joyce, the original creator of the Guardians. Most of the backlash against the film is caused by it, as there was no such hate until after the ship took off.
During an Instagram Q/A, William Joyce was asked about the ship to which he replied that "It would be delightful if they could be together", while also acknowledging the impossibility of it as the two are owned by rival studios. While met with approval, he was also on the receiving end of some not so favorable comments and he hasn't touched the subject ever since, at least on a public forum.
Dark Chocolate or Rotten Eggs for Pitch/Bunnymund.
Black Christmas and White Christmas for Pitch/North and Jack/North, respectively.
Eggnog for Bunnymund/North.
Cavity for Pitch/Tooth.
The Black Ice ship has been expanded to include Golden Frost for Kozmotis/Jack.
It's the Same, Now It Sucks!: A major complaint against Jack Frost is that despite his interesting character and origin, he still acts like a typical DreamWorks male lead: cocky, smug, and a smartass, yet still an outsider we are supposed to identify with and feel sorry for. Not helping is him being voiced by a A-lister, rather than a more proper voice for his supposed age.
Those who dislike Jack Frost, specifically Elsa fans who hate having him paired up with her, love to bring up the movie's poor box office performance and being a dead franchise. Many fanarts and jokes will tend to portray Jack as a failure who's trying to date Elsa as a way to save his career.
Jack Frost will often be shown as being this to Shrek, a fan favorite and Dreamworks darling.
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.
As a snarky response to the fans' preference for Jack's spirit form over his human appearance, many say that Jack is a "Hollywood white-washing" that everyone is okay with.
Pitch's interactions with Jamie and the other children towards the end of the film can come across as molester-ish, especially the tone of voice with which he taunts and threatens them. This vibe is also present in the scene where he turns Cupcake's unicorn dream into a nightmare and voices just how much he's getting off on her fear.
Word of God mentions in the official DVD commentary that people accused Guardians of ripping off The Avengers, though they claim Guardians was in production long before Avengers was even announced, due to animation taking longer to create than live-action. (And that's not even getting into the books, which are older than the movie.) However, The Avengers as well as the MCU as a whole, was years in development (the first Iron Man movie was released in 2008) and it was based on a long running series of comics that were around decades before William Joyce began work on his books. Outside of the superhero team, there are barely any similarities story-wise. The film and books as a whole owe a lot more to the Rankin Bass Christmas specials than to the superhero genre.
Many fans think of this movie as being the first film to feature Jack Frost as a young protagonist. That honor actually belongs to the stop-motion Rankin Bass Jack Frost film.
The spirits in this movie are invisible to anyone who doesn't believe in them. One of them can easily watch you from the window or stand right next to you as they have means to get into your house, and you will never know. They can even access your dreams and memories.
The Monster Under The Bed is real. He can corrupt your dreams and give you endless nightmares about things you fear the most or, if you have children, he is likely to target them. And there's nothing you can do about it.
Pitch gets this treatment as much as a villain can. In the movie, he's a bad guy all right, but he is somewhat morally ambiguous. In some fanfics, he turns into a Voldemort with a nose.
Ship-to-Ship Combat: A rather odd crossover example, but fans who ship Jack with Rapunzel sometimes get into disputes 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.
They Wasted a Perfectly Good Plot: A major driving force for Jack was his desire to know about his past and his reason for being. However, when he actually regains his memories, the revelation of him having a family and a sister seems to have no lasting impact on him after the reveal. While it's true that 300 years have passed since he became a mythical spirit so his family would be long dead, the story doesn't show him having any desire to explore his past life. (It's possible they were planning on exploring this in future sequels that never launched due to this movie being an Acclaimed Flop).
Uncanny Valley: The design of the Guardians and some of the kids comes across as this to several viewers. This can be attributed to the fact that the animators were trying out a new skin texture that, as the Art of the Rise Of The Guardians book described, resembles "jelly".
Vindicated by History: The movie bombed in theaters but quickly developed a sizable, dedicated internet fanbase. Then, thanks to the superhero genre's popularity (especially to The Avengers coming out that same year), and the perceived similarities between Jack Frost and Elsa, many people who missed Rise of the Guardians in theaters checked it out and came to appreciate its merits. Still, DreamWorks continues to treat the movie as an Old Shame.
Visual Effects of Awesome: Everything is detailed to the last hair. The design of the ice and snowflakes Jack creates perfectly captures the art style of Guillermo Del Toro.
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, causing the children to lose their belief in him.
Pitch, too, (albeit of the Jerkass variety), once you know he went 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, causing her palace to disintegrate and 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...
Jamie Bennett, when all the kids start losing their belief in the Guardians.
Jack's sister, who witness his death and didn't even know he became a spirit.
What an Idiot!: While it's understandable that Jack was tricked, it's still somewhat short-sighted to follow a mysterious, yet familiar voice into a dark forest.