Crazy Awesome: Barley. If using a D&D-style board game to teach his little brother magic, using a children's coloring menu as a treasure map, and using an enlarged cheese puff as a raft doesn't qualify, nothing does.
Two brothers trying to resurrect their dead father and failing? Heartbreaking. Two brothers only managing to resurrect the lower-half of his body? Hilarious.
The fight with the pixie bikers. If the bikers were human-sized, the scene would be terrifying and likely bump up the movie's rating. The fact that the bikers are tiny pixies pushes it into the realm of absurdity.
The mermaid in the kiddie pool, who is prominently shown in the teaser, which translates to a short cameo in the movie's prologue. Not only does she have outlandish theories surrounding how she even got there in the first place, but she is also relatively uniquely designed.
Fandom Rivalry: With Brave, Pixar's previous attempt at a High Fantasy setting. Fans are at odds over which one is considered better, given that both have similar themes of family and appreciating the things you have right now.
Fanfic Fuel: The film ends on a rather light-weight note of And the Adventure Continues with regards to the Lightfoot brothers. However, it leaves some very open-ended questions on how society will deal with this potential resurgence of magic or that a high-school student now has access to some incredible powers or that a van is now flying over New Mushroomton. It also leaves other kind of questions, like what if others joined the boys on their journey?
Fan-Preferred Couple: Laurel/Corey is gaining plenty of traction, due to their great chemistry together and the fact that Laurel's canon relationship with Colt has so little impact on the plot that it might as well not exist.
Common descriptions for the film include "Zootopia, but with mythical beings instead of furries".
Specter was reported as Disney's "first" openly LGBTQ character, the seventh character that's been promoted as their first, leading to people announcing themselves as Disney's next "first LGBTQ character," wondering who will be Disney's eighth "first," and proclaiming other Disney and non-Disney characters were the real first.
As soon as Specter's most prominent scene was available online, it became common practice to dub her over with memetic audio, such as here.
Misblamed: Disney has taken flak for presenting Specter as their "first LGBT character", even though there've been others before hernote Most notably, LeFou in the 2017 remake of Beauty and the Beast was also hyped up as "Disney's first gay character"... which ended up amounting to only a few minor Ho Yay scenes and a one-off gag at the very end of the film. First off, she's a character in a Pixar film but it is the first time a Disney/Pixar character is explicitly stated to be LGBT in the film's dialog rather than presented as Ambiguously Gay and confirmed by Word of Gay later. However, the reveal of Specter having a girlfriend was never meant to be groundbreaking, as her voice actress, Lena Waithe, improvised the girlfriend line in order to give her character more depth and Disney didn't go out of its way to publicize they were presenting anything unique about the character.
Moe: With his poofy hair, giant elf ears and shy personality, Ian is just adorable.
Narm Charm: The boys being left to carry around their father's sentient but disembodied legs so Crosses the Line Twice-level of hilarious that one wouldn't be blamed for wondering how effective the more emotional scenes of it will work out. Though the film takes full advantage of it for a slew of comedic moments, not only are the emotional scenes not lessened by the situation, but the fact that Wilden can't communicate verbally even heightens them. Special mention goes to the scene wherein Wilden legs confirm that he's with his sons when Barley drums on his feet and he reaches out to press his foot against Ian's.
The idea of a van crashing being played off as a Heroic Sacrifice while being filled with visual symbolism of a noble steed galloping off to Valhalla sounds absolutely ridiculous... and yet somehow the film manages to pull it off as a genuinely tearjerking moment. Pixar can somehow make you cry for a suburban vehicle.
One True Threesome: For those who don't mind Laurel/Colt but enjoy the former's interactions with Corey, Corey/Laurel/Colt might be your ticket. And it's not as widespread but Corey/Laurel/Wilden is something that could have been a thing pre-movie plot.
Laurel and the Manticore having to retrieve the Cursecrusher from a pawn shop only really serves to delay the two a little more. While the two end up playing a large part in the climax of the film, nothing would've been lost if the Manticore simply still had it in her possession.
When our protagonists find themselves back in town, Ian chews Barley out for leading them on a wild goose chase. Trouble is, the movie had already had similar scenes (when they got stopped by the cops and disguised themselves as Colt, causing Ian to inadvertently reveal that he sees Barley as a screw-up, and when they encounter the raven statue which made them realize that "Raven's Point" is meant to be taken literally), causing this scene to feel like the movie's repeating itself.
Squick: Try not to think too hard about how Laurel, who is a human-sized elf, would be intimate with her boyfriend Colt, a centaur.
Colt. The movie implies that he has a hard time adjusting to being a father to two teens, but none of the interactions he has with the brothers go beyond them not really respecting him and feeling uncomfortable around each other. In fact, removing any reference to him and Laurel being together wouldn't really change his character beyond him being the clichéd authority figure whose only role in the plot is being an obstacle for the main characters.
Specter was heavily advertised in promotions for the film, leading fans to assume she would be a main character. She ends up having only a few moments of screen time. Many were left upset about this, especially after the hype that she was openly gay in the film (which amounts to a single line about her girlfriend).
They Wasted a Perfectly Good Plot: While the main focus of the story is Ian and Barley's quest to revive their dad, it was set in a fantasy world where technology has replaced magic and it would have been interesting to show how such a society is different from ours. For example, if there exists a Manticore that could breathe fire, why doesn't her restaurant have any water sprinklers in case of accidents? How do homes and vehicles owned by centaurs accommodate their equine bodies? How do mermaids get around on land? How do tiny pixies compensate for living in a world designed for bigger people? There are quite a few ideas that could have been explored to flesh out the world building.
The film's prologue of wizards coming to terms with how they're not needed anymore would make an interesting movie in its own right.
Tough Act to Follow: Audiences have pretty much agreed that, while the movie doesn't hold up as much in comparison to other Pixar movies that came before it, it's still a good movie in its own right.
Trailer Joke Decay: The exchange of Barley saying, "Dad was a wizard!" and Laurel replying, "Your dad was an accountant!" appears in an ad that was in heavy rotation on YouTube. Understandable because Disney was heavily pushing their Disney+ streaming service during the "shelter at home" caused by COVID-19 and Onward was one of its prime offerings during that period. However, the extensive repetition of the exact same ad did drive the (already weak) joke into the ground.
Unexpected Character: The film is set in a fantasy world populated with centaurs, elves, pixies and the like. Barley even plays a D&D pastiche called "Quests of Yore" that he insists is historically accurate. Pixar goes through considerable effort to establish that this world is a loving homage to Dungeons & Dragons that it's a surprise and delight to see an actual D&D monster appear like the Gelatinous Cube.