Director Dean DeBlois has called this movie The Empire Strikes Back of a proposed trilogy, but that's a bit misleading. For one thing, this movie can be enjoyed just fine in a vacuum, and it ends conclusively without a cliffhanger. What it does have in common is a heightened sense of stakes (there is a definite "shit just got real" moment), a period of time where our main heroes separate into two separate groups, a suddenly not-dead parent, and a continuation of the theme that "size matters not." Additionally, handicap matters not - Hiccup will likely be a great role for actual amputee children, though he's no less of one to everyone else as well. The only storylines left unresolved are the movie's brief dalliances with young romance - Ruffnut (Kristen Wiig) is lusted over by both Snotlout (Jonah Hill) and Fishlegs (Christopher Mintz-Plasse) but only has eyes for bicep-heavy Eret. Presumably she'll choose her man in part 3.
So why does the 3D matter? It's not just a plus in scenes where dragons swoop through the sky, but is crucial to establishing a sense of scale - you see just how small Hiccup and Toothless are (and feel) in the face of a much larger canvas. Later, when Stoick and Drago do battle in the foreground while two kaiju go at it behind them, the tricks of perspective that make the Vikings look bigger and more important than they'd otherwise be as antlike creatures underfoot is an important stylistic choice, enhanced when actual depth can be added.
The How to Train Your Dragon movies are rare examples of Americans learning from anime without copying it - all too often we get the reverse, with big eyes and angular features substituting for story and character. Hiccup and friends don't look like the Japanese cartoon style in any way, but they are drawn in a comedic, exaggerated fashion while being placed into an adventure that overall is anything but. My personal preference would be for something more realistic, even live-action, but it is what it is, and the story still ultimately sucked me in. I think it's in danger of being a tad overrated - ultimately, it's still a pretty standard chosen-one/underdog hero formula - but there's no denying it's fun, and it's diverse enough that people can't complain it doesn't have enough women (plus it has a Latina playing a Scandinavian - color-blind casting!). About that supposedly gay character, though...that whole thing hinges on how you interpret one vaguely worded line about him being unmarried. The film never says he is or isn't.
It's enough to make me believe that maybe the time is now for Disney to try The Black Cauldron again, and do it right this time.