Violent content: humans and dragons fighting each others (very mild visualization)
Sexual content: two kisses, if I didn't lose my count
I love the fact that this is a family movie. Although Hiccup and his friends have grown up and--as expected--crushes and kisses are obvious, they are just side dish served as humor material.
The plot is first about Hiccup's finding his true calling, second about how dragons and humans should co-exist. You may think that's also the case with the first installment, but trust me the sequel will touch you in a different way.
Make sure you watch it in 3D for a satisfying ride. (I watched it in 2D and now am longing to see it again... in 3D.)