These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Angst? What Angst?: Hiccup once a book nearly gets murdered, eaten, drowned, and at worst gets nightmares later in life as an old man. He remains chipper about triumphing over ordinary Viking life, at least until Book 9.
Ensemble Darkhorse: Camicazi, for being a Badass Adorable, taking on several grown swordsmen on her own, and acknowledging that Hiccup is "not bad for being a boy". Some fans wish she had been added to the movie.
Evil Is Sexy: Alvin before he lost his hair got this treatment in-universe. He was quite a charming with the ladies apparently.
Any mention of Grimbeard The Ghastly, as well as his son Hiccup Horrendous Haddock II, Hiccup III's ancestor.
The Dragon Whisperer's way
Grimbeard's last letter, regretting how "treasure" tore his tribe apart, hence why he hid it. In How To Break a Dragon's Heart we learn thanks to the witch that Grimbeard also allowed a jealous son named Thugheart tear his tribe apart because of envy for Hiccup II.
Iron Woobie: Valhallarama of the White Arms in ''How to Betray A Dragon's Hero."
The Scrappy: Snotlout eventually earns this status in-universe after he betrays Hiccup one time too many.
Unintentionally Sympathetic: The Hairy Scary Librarian. Sure, he is Ax-Crazy and hoarding all the books in the library for himself and killing anyone who tries to steal said books, but he gives the thieves fair warning to return the books before he slices them up, and gets sent to the Amber Slavelands because Hiccup frames him for stealing a Murderous Stealth Dragon. And when the witch lies to him about receiving freedom, he switches sides quite easily after settling his score with Hiccup.
YMMV for the film version of How to Train Your Dragon:
Adaptation Displacement: Due to the film's enormous popularity, the book isn't very well-known about in comparison.
After the final battle, Hiccup has but a few moments of remorse upon waking to find that he's lost his left foot and been given a metal replacement. But forget angst— there are dragons outside and they are now a part of the Viking village! There's also the fact that he lost his foot in battle like a real Viking — which he had wished for all his life before befriending Toothless. Finally, he gets a big kiss from Astrid meaning he has the local Action Girl as his girlfriend.
Mixes with a bit of Book Ends as well as Hiccup first helped Toothless deal with his missing tail fin, and now Toothless returns the favor by helping Hiccup learn to walk with his artificial foot. Considering that he almost died, losing a foot probably isn't that big of a deal to him at the moment.
The Tale of the Boneknapper which chronologically takes place after the events of the movie showcase Hiccup walking and running just fine on his prosthetic leg which means that he has largely adapted to his injuries.
Award Snub: It came out the same year as Toy Story 3 and got overshadowed come award season. To the very least, it did come out on top for the Annie Awards (to the pleasure of its fans and disdain of Pixar fanboys.)
Test Drive. Chills every time. If there was ever a piece written about the thrill of flying, it's Test Drive.
Really, the whole soundtrack by John Powell can count here. It helps that key scenes were set to highlight the music in parts like "Forbidden Friendship," "Romantic Flight," and "Where's Hiccup" rather than with dialogue, often to great effect.
And for further proof of its epicness, NBC Sports used "Test Drive" for its recap of Rory McIlroy's historic 2011 U.S. Open run (setting the lowest score in the major's 100+-year history) during its coverage of the golf event.
In-Universe example: Vikings are shown to be ready and willing to fight just about any dragon alive, sometimes very enthusiastically. And yet, every single viking will load their pants the SECOND they hear the shriek of a Night Fury.
Inferred Holocaust: Although not seen, it's stated outright that the casualties for the Viking/Dragon war in general were large.
Stoick: They've killed hundreds of us! Hiccup: And we've killed THOUSANDS of them!
While the Red Death didn't get the chance to wipe the Vikings out, we see many of them are very close to the ships as it sets them fire, which should have killed a number of them, but all we see are some falling into the water.
Some Anvils Need to Be Dropped: Violence isn't always the best solution, parents don't always know what's best for their kids, and just because something is a tradition does not mean that it is necessarily right.
When Hiccup wakes up and discovers that he's lost his left leg in the battle; it's even more jarring if you didn't hear Gobber imply it earlier. Not to mention, the part Hiccup tries to walk using his artificial leg and immediately stumbles is like an emotional kick to the groin.
The scene where Stoick disowns Hiccup. Hiccup pleads for him to listen for once in his life, is thrown back and told "You're not my son". Hiccup's face as he watches his father leave really helped the tears flow, as does Stoick's expression after the door closes behind him.
Stoick mourning over his son's apparent demise in the aftermath of the fight with the Red Death. Granted, we all knew Toothless saved him somehow, but the sheer despair and sorrow in Stoick's voice as he grieves over the loss of his only son feels like a punch to the gut.
Test Drive, strangely enough. There's just so much adrenaline and emotion that it gets kind of overwhelming.
The Terrible Terrors remind one of hyperactive, firebreathing chihuahuas. Enter the awwwww factor when one of them snuggles under Hiccup's arm and purrs itself to sleep there. A bit of trivia: That particular Terror was voiced by Paco the Chihuahua, after sound designer Nia Hansen of Skywalker Sound saw a video of him on YouTube and paid his owners $100 for Paco's voice: video 1, video 2, local news articlearticle 2.
Gronckles, especially the one Hiccup tames with the dragon-nip and the poor little guy that gets eaten
Viewer Gender Confusion: Quite a number of viewers thought that Toothless was a girl when he first appeared. Unintentional feminine symbolism shows up around Toothless throughout the first film, such as Toothless looking at a bird's nest with eggs in it and holding onto an unconscious Hiccup like a dog or cat would cradle a newborn puppy or kitten.
Also, the Red/Green Death. Due to Astrid's comparison of it to a beehive's queen, almost everyone refers to it as female. According to the books and Chris Sanders, the dragon is actually male, though there's really no indication one way or another in the movie.
The Woobie: Hiccup. Before Toothless, he's basically friendless, motherless, viewed as huge disappointment by his dad, bullied by his peers, ostracized by the rest of Berk and a failure at everything Viking society values.
Toothless for losing his tailfin and ability to fly, and possibly being the The Last of His Kind. (Or at least the only Night Fury among all the other dragons.)
YMMV for the DVD short film Legend of the Boneknapper Dragon:
Nightmare Fuel: The Boneknapper Dragon is a huge dragon that wears stolen bones like a giant flying skeleton, and will stop at nothing to find the perfect bone for its macabre coat of armor. Also, without a certain bone on its armor, it can gain up on unwary vikings since it can't roar.
Ugly Cute: The Boneknapper dragon doesn't hesitate to warm up to Gobber and even allows him to give it a chin scratch when he gives it back the bone it needed to complete its armor and allow itself to roar.