There's not really any grand reward for completing or even mastering a song in DDR. Your performance is graded, and usually the only unlockable things are new songs, if any.

You dance for the fun of it. Whether you're playing casually, acing your favourite song or completing the whole track library, you dance because you become one with the music. When you play, you fly through the arrows as your only focus is on the moves you make and how well you fare. Your feet to the rhythm. And once you finish one song, you step it to the next.

So step it up,

and dance, damn you!