While the video I shared above cuts off the very beginning, the rest of it is pretty much solid. If you were not interested after watching the first well-done 10 minutes of the movie, then I'm not sure what will impress you. The music, transitions, choreography, monologue and dialogue, and action are excellent. It also serves as a good preview on what the rest of the film will be like. I theorize that if you make a good opening, you shouldn't need a spoiler-filled trailer to build up hype.
You have a quick introduction that is timed perfectly with fantastic music. In fact, I often listen to the soundtrack and reflect on well-done it was. How to Train Your Dragon does fantastic job by starting with a nice mellow but vikingish tune and scene but quickly turns to humor and epicness. Within the first 10 minutes, almost all of the themes that will be apart are covered. You are subconsciously told that there will moments of reflection, humor, epic encounters, romance, and a few twists. The only theme missing that is the core to the whole movie which is friendship.
As sort of a proof of how well the opening follows the movie, consider which characters get developed in the film. There are three: Hiccup, Stoic, and Astrid. They not only appear in that order, but their development and time spent as the focus of the movie corresponds to their time as the focus within the first few minutes. Even though Gobber is present, he's never the real focus. You know this because Gobber is used to tell us more about Hiccup (and later about Stoic or dragons) but never about himself. However, Stoic has a few scenes where he is the star of the scene and Astrid has one moment in the opening as well where she's the star. Now if you argue there's a fourth character, I would agree if you mean all of dragons. This is because they also take the focus as Hiccup informs us about a few of them which happens quite a bit throughout the film but at a good pace.
While it may take 10 minutes to present this solid picture, the fact that it is pleasant, entertaining, and well executed makes you forget about the time. I kept thinking it was 5 minutes only because of the time on the soundtrack, but when I actually analyzed, I found it was closed to 10. I think it's safe to say that if we can make the audience forget about the time, then we have done a good job with our art. And if the audience is pleased after finishing the whole thing, then we did a fantastic work of art.
J. D. Nyle