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?: Averted rather beautifully with Hiccup. He still has some lingering self-esteem and image problems that he had in the movie. Seeing as he was pretty much ostracized by his entire community up until recently, this is very understandable. Played straight, however, with Hiccup seemingly having no issues whatsoever with his prosthetic foot. Any comment he makes about it is incredibly snarky, but with no trace of angst.
Considering that most of the adult men of Berk have at least one prosthetic, this isn't all that surprising. Losing a limb seems more like a mark of pride than anything else in their culture.
Awesome Animation: The animation is at times on par with the original movie, which is quite impressive for a weekly animated series.
Awesome Music: Most of the leitmotifs from the original film show up in series, with several new arrangements of each appearing.
Die for Our Ship: Heather received hate before she even debuted. When it was said that she might "threaten Hiccup's love for Astrid", the shippers went nuts. This became Hilarious in Hindsight once she actually showed up, not only in that she treated Hiccup more or less like all the other boys, but in that "Heather Report 1&2" is more about Heather's relationship with Astrid than her relationship with Hiccup. Not to mention when Heather is leaving Berk she totally ignores that Hiccup clearly has his arms open for a hug, just saying goodbye to him while still hugging Toothless and Astrid. Really, she's more likely to interfere with Hiccup/Astrid due to Les Yay with Astrid than stealing Hiccup away!
Some shippers are also like this with DagCup, the Dagur and Hiccup ship. Poor Astrid usually loses her Character Development so that the Berserker and Hooligan's relationship can exist.
Draco in Leather Pants/ Evil Is Sexy: Dagur gets this treatment in fanfiction, usually to justify why he is psychotic and to make him appealing. The fact that he's the youngest villain, not to mention the most competent, wins the crowd quite a bit.
Foe Yay: Between Dagur and Hiccup just about every time they interact, starting in "The Night and the Fury"; lampshaded by the latter as being rather uncomfortable. Indeed, at the end of the episode, Dagur seems more upset by Hiccup personally betraying him than Berk's possession of domesticated dragons. Just look at Dagur's behavior as well! He wanted to spend time with Hiccup and gets annoyed when his friends keep showing up, is angry upon learning of the bond between Hiccup and his bond (implying clingy possessiveness), and mentioned he couldn't stop thinking about him ever since the Berk incident. He wants Hiccup to "kiss his boots" in one episode, and obsesses over it.
Anything Bucket does, once you find out in "Portrait of Hiccup as a Buff Man" that he acts the way he does because a dragon ate half of his brain. Although this was actually implied from the first episode, when part of one of Mildew's anti-dragon diatribes involves pointing out that Bucket took the head injury that requires him to have the bucket welded to his skull as a makeshift plate from a dragon.
Astrid's zeal in being a dragon slayer in the movie. Just her being a typical Berk Viking and actually being competent at it, right? Now consider how her uncle died and the effect it had on her family's honor and suddenly it takes on a chilling tone. She is unlikely to be the only case like that.
The villagers' excited chanting of "Viking Funeral" since Stoick's in the sequel.
Idiot Plot: Tuffnut spend an entire episode trapped in a net with Barf and Belch nearby all because nobody could think of the most obvious solution to get him out: have one head simply bite or burn the net while the other head is placed under him for him to sit on.
Just Here for Godzilla: Face it, at least a fourth of this show's audience is just watching to see the new dragons.
Mildew crossed it in "Alvin and the Outcasts". Getting the dragons exiled was bad enough, but then he intends to sell Hiccup out to the outcasts. And just to make sure the viewer knows this isn't a "needs of the many" kind of choice, he gloats about it to Astrid.
Alvin himself crosses it by threatening to kill Heather's parents to force her to be The Mole.
It's heavily implied Dagur crossed this a while back by killing his own father. With an axe, no less.
Mildew wasn't exactly well-liked considering his desire to get rid of Toothless and the dragons, but the events of "In Dragons We Trust" where he frames and succeeds in exiling them has driven the last nail in. Then the hate for the guy skyrockets with each attempt to restore the status quo at the expense of the dragons, such as trying to poison, which could kill, the dragons. Heck, he's even The Scrappy In-Universe!
Bucket: It took us three days to dig Mildew out!
Mulch: And the rest of our lives to wonder why we bothered...
Snotlout's dad may have joined him. Outright threatening your kid for the possibility of not winning a game (one that the kid has won plenty of times before, to boot), and the sheer panic Snotlout displays when he thinks he'll lose (indicating that his dad was totally serious), has made him pretty divisive. He got a Karma Houdini too, since Hiccup just let Snotlout win instead of addressing the real problem with someone. Then he has a Kick the Dog moment in "Race to Fireworm Island" when he advises Snoutlout to abandon an ill and dying Hookfang and find another dragon.
Snotlout himself isn't well-liked in-universe, either, to the point the village cheered when they thought he died in battle in "Cast Out, Part 1".
Special Effects Failure: Though the animation is pretty good, the series tends to stumble in action sequences, particularly dragon breath. By watching episodes like "The Terrible Twos", it's obvious they put extra effort into the fight scene near the end of "Alvin and the Outcasts".
The physics of the show are quite schizophrenic; some episodes use full-on motion capture for the human characters, while others use stiff-looking manual rigging for all motion. Watch an episode such as "Portrait of Hiccup as a Buff Man" with this in mind, and it becomes difficult to take your mind off the blocky and robotic motions of the characters.
An especially jarring thing about the animation is the hair. Hiccup's hair almost always gets a lot of detail, while the other's don't. So you go from a closeup of Hiccup riding Toothless and his hair is moving just like it would in real life, and then you go to a close up of the twins and their hair (which is much longer than Hiccup's) is separated into three chunky blocks that move very unrealistically.
In-Universe example. Gobber decides to re-name a baby girl "Magnus" because he thinks it looks more like a boy.
Snotlout also has trouble telling which of the twins is supposed to be the girl sometimes. Not to mention many viewers keep confusing who is named Ruffnut and who is Tuffnut. Ruffnut even gets her hair cut, which would have helped clear the confusion up, but it is back to normal by the next episode.