I think the hype is simple, we don't know too much on the quality of the story or gameplay so the hype is based solely on the visuals and the potential of the game. If you love Japanese games then there's a decent chance you grew up with anime, and if you grew up watching anime you probably love at least one of the films by Studio Ghibli. You may not love them all but there's probably one near and dear to your heart (for me personally that's Nausicaa even though it was before the studio formed it had a lot of the same people). Ni no kuni is looking like it really makes use of the collaboration between Ghibli and Level 5 to make something that looks like playing a Ghibli film, and fans of their movies can't help but get excited at the prospect. Even if you aren't particularly a fan of one of the movies it's hard to argue that the game is ugly, it really is quite beautiful.
Now this all means that the game has the potential to feel like playing a Ghibli movie if the story can match the visuals. And honestly if it had a story that matched a Ghilbli movie as well as the visuals we've already seen I wouldn't really care if the battle system isn't all that amazing. But we'll just have to see.