Plot: Hiccup (really his name is Hiccup) tries to maintain the peace as a rival dragon master tries to capture all the dragons. On the way he meets a surprise character, who gives him advice. In the end there is a big battle with flying dragons. [imdb] [photos]
The basic dramatic action was never that strong. They tried to set up parent-child tension, but that never worked -- partially because Hiccup is just to cynical to show real emotion. Similarly the interaction between the Hiccup and his friends just isn't strong enough to be entertaining. When the surprise character is introduced -- it is interested for a while, but the movie slows way down while it processes the plot change: sleep inducing. The final battle was not dramatic enough for various reasons.
The flirting between Hiccup and Astrid was fun though. The stud-infatuated Ruffnut was a running gag that worked too.
The dragons were comic and not cool. The animated people was great however - especially the hair. Computer animation of hair has really advanced. The sound track was also good.
They deliberately gave Hiccup and his friends modern sounding dialog whereas most everyone else had a stilted, pan-britania accent. This heightened the sense of irony and decreased the dramatic tension.
The first movie had a problem with joke-y dialog about family members getting eaten by dragons, and way too high a body count for such a light tone. This movie had a lower body count, but was no fun.