Big combos get you extra XP in story mode, and are a necessity for earning medals in challenge mode. (n4g.com)
The difficulties keep you coming back to each level since you have to earn all medals on a difficulty before the next is unlocked. (xboxgamerreviews.com)
Depending on how long you spend in a level along with how many jumps you do in each level earns you a medal at the end of the level. (thevitalounge.net)