Marc Craste does an interesting interview:
When you have characters with masks on, that can do very little (facially) other then be threatening. And then you have the heroes who can't do very much more, and yet they can cover a range of emotions simply because they can move their eyes. It's all in the eyes.
Not only are the are a lot of good economic reasons for simplyfing design choices. I think very often animation works when it is simpler, the more complex it gets, the more your asking the audience to believe, the more difficult it is to convey a deep emotion. Whereas when they're little cypher's like this, it's amazing the amount of people who shed a tear over this. These are little characters who can't do much other then look bewildered, in love, or bewildered. So it was a break from any desire to go into any complex design, and try and keep things simple. The most important thing is to have some kind of endearing quality to the characters, so you do actually care regardless of whether its talking or whatever it's doing, a lot of that has to do with the worlds I build around my characters, they all tend to be little and lost in a great big threatening world.
*interview found by Lango