I took my RL crew to see the 2D version (older DD gets sick in 3D movies). Even so, it was thoroughly watchable and enjoyable.
Spoilers alert!! Highlight the "empty" section below to see it.
Hiccup is a misfit Viking, physically weak but very intelligent, and a tinkerer . . . oh and he's the chief's son. Instead of killing a dragon when he has the opportunity, he befriends it, names it Toothless, and discovers that dragons are not the enemy that the Vikings think they are. Meanwhile he ends up enrolled in the dragon slaying school and uses his new-found knowledge to subdue the dragons nonviolently and rises to the top of the class, much to the irritation of his rival and romantic interest in the story, Astrid. Inevitable misunderstandings ensue he is disowned, and he and his pet dragon end up saving the village and becoming heroes.
I really liked the characters' portrayals. They came alive for me and I cried at their heartbreaks and rejoiced in their triumphs. The animation was phenomenal, I was amazed at the textures of their clothes and the way they moved. CG animation has come so far in the past decade. I can't wait 'til the artists finally overcome the uncanny valley as it applies to CG characters.