If you collect all the Lums on a level (which is not easy!) then you unlock a special reimagined version of that stage where the layout and positioning of bad guys is different.
Rayman Fiesta Run introduces the concept of bosses, which you need to defeat when they pop up at various points of your journey through the game. This adds an extra dimension to the gameplay and injects even more fun. I especially liked the giant piranhas that chase you through the level in a not-so subtle parody of Temple Run.