I think this movie had tons of potential because the world was awesome and the animation was gorgeous. I think the biggest issue was the pacing (lots of side characters were introduced but there was any time to get to know them). The comedy (particularly Sisu and the baby) fell flat to the point that I think the movie would've been better without. I found Sisu in particular to be pretty obnoxious at times. Others have commented on her modern way of speaking removing the viewer from the fantasy world, and I kind of agree.
I think that this story would really shine if it were a series of shorter episodes instead of a movie, so there was more time to explore each location and character.