All-Star Cast: If you've taken a look at the character section, you will see why. Averted in that it's a lot of recognizable names, but no overwhelming star power, as apparently finding the perfect voice for each characters was more important than who to name-drop on the adverts.
Approval of God: Cressida Cowell, when asked, showed no offence or anger at the differences the movie had from the original novel (tone, setup, etc.)- she acknowledged that it worked better for the new medium, and that the story stood strong on its own despite not adapting the books' narrative more strictly.
Cowboy BeBop at His Computer: A whole series of them, concerning the giant dragon at the end of the movie. In-movie, it's only referred to as the Queen Dragon. Then the soundtrack names it the Green Death, obviously a Shout-Out to the original novel. Then the McD toy referred to it simply as 'R.D.' A review subsequently named it the Red Death. The video game for the the Nintendo DS mentioned it as the Red Death. Wordof God (in the DVD commentary) officially named it the Red Death. Nonetheless, it's still commonly referred to as the Green Death by consensus in the entry, in honor of the original book.
And yet, the dragon in question is blue in the movie... (though to be fair it has some amount of red coloring).
Doing It for the Art: Tons of information on the Dragon wiki is dedicated to the dragons' various characteristics. This includes strength and weaknesses (Monstrous Nightmares are gifted with exceptionally good vision and have extend-able wings that allow it to intimidate enemies, but its large wings also render it slow-going on land), wing power (Night Furies are the only dragons capable of vertical liftoff), their personality traits (Deadly Nadders are extremely vain, evidenced by the generous amounts of time spent preening themselves), fire-breathing abilities (the Night Fury has an unconventional fire attack involving semi-solid mass set alight with an acetylene/oxygen flame), eggs (Gronckle eggs are the smallest of the species, barely larger than a marble), egg-laying habits (Terrible Terrors lay their eggs in the cracks of tree trunks) and even training tips.
Fan Nickname: At the Sticks and Stones forum, the posters there have nicknamed the Red/Green Death the "Christmas" Death since no one really knows whether it is "Red" or "Green" since both movie and book disagree on that issue.
Sleeper Hit: Even though the film debuted at #1 in the box office, its premiere weekend take was judged a disappointment and people hastily wrote it off as a de facto failure. However, the film developed such ecstatic word of mouth that the film just kept going and going until it arguably regained #1 in week 4 (the official grosses put Kick-Ass on top by a few hundred thousand dollars, but Lionsgate counted Thursday night grosses into its weekend total, therefore indicating the possibility that Dragon was the actual winner) and definitely in week 5; it's a box office performance path that almost never happens nowadays! So, the "disappointment" became the spring pop culture sensation in 2010, finishing a shade under half a billion dollars worldwide when all said and done.
Both the lines "dat da da, we're dead" and "thank-you for nothing, you useless reptile" were ad-libbed by Jay Baruchel. Other lines were also improvised, including much of Snotlout's dialogue in the scene where Gobber tells the teens how he lost his limbs.
The shot of Toothless opening his eyes after his wing temporarily blocks them from view was actually an animation mistake, but was kept in because it lent an eery quality to the rest of the scene.
Crossing over with Executive Meddling, the film was originally going to be very close to the book, considerably Lighter and Softer than the end result; concept art shows a much younger Hiccup, Snotlout as a girl, a much smaller Toothless, Astrid with huge braids giving her a Helga look, and a lot more gags. Hiccup's mom was going to be in the film and act as a voice of reason, as she does in the second film. The directors thought she took away from Hiccup and Stoic's relationship so they cut her out. She would later debut in the second film.
Although Snotlout was male in the original books, meaning that in that case they averted using Gender Flip - most likely when they decided to create Ruffnut, thus give the group more gender balance without making Snotlout a girl.
Toothless' design was originally meant to be more canine based, but it was changed to be feline based instead after one of the directors saw a screensaver of a black panther.