I agree, that would be one of the hardest parts... Allomancy is, in the first book, basically nine different powers. And most of them would be incredibly difficult to portray visually. Pewter doesn't have any visual effect, except perhaps in their posture. Iron and steel would probably be the easiest. They might almost have to just have Kelsier explain to Vin, "our powers are based on metals. By burning metals in our stomach, we can strengthen our bodies, influence others' emotions, and push or pull on metals around us." Then they could have some beautiful visuals of super-enhanced jumps by pushing coins into the ground. They wouldn't have time to explain everything--what gold does, the subtleties of bronze, and so on.
Actually, I think Allomancy can be done via colour association. We can have faint auras on the characters when they burn metals. Something like pewter, I envision a purple aura, which doesn't necessarily need to be on the entire time (for the purpose for the movie), but you get an flash of purple when someone is attack or regaining balance etc.
In terms of pushing and pulling, the blue lines would be needed clearly. However, they can single out directions after the Misting chooses a line.
Penny for your thoughts. FYI - I've only read the first book so I don't know if there is more to Allomancy or not.