Rewards and Points
I was curious besides the inherent benefit of learning do points and level matter? I'm not sure how duolingo could implement this or what the rewards would be, but it seems duolingo has created his "game" type learning atmosphere, there should be something to spend those points on. Perhaps some kind of challenge mode where you could complete against another member of the site.
In my group of family/friends, we use it as a way to keep track of score, to see how well people are doing relative to each other.