First, the animation is really spectacular to me.
The animations of the dragons, taken from real world animals - lizards, cats, birds and the like bring realism (well, as best as you can with an obviously animated movie).
Second, I love the whole viking motif. Yes I know LOTR and skyrim have drawn heavily from that pool. Vikings and Dragons never gets old for me.
It may be a little silly, but its fun, and my kids like it too.