World wide Leaderboard
It would be cool if we could see our global ranking. In my case, it would motivate me to keep on pushing further.
Maybe I'm a pessimist but i don't see how that would motivate me at all. A global leaderboard would only say something like i am 1,459,520nd in the world or another really high number (I don't know how many people use Duolingo). People would probably look at this and think "look at all these people that are better than me". It would certainly damage my motivation. It is better to keep it among the people you friend.
Ok, yeah, thats one way to view it. In my case, it would have the opposite effect. But it's nice to view different points of view.
It would be full of unreachable scores that would make me sad.
Also, since we have a finite goal of finishing the tree (learning a language), score doesn't mean anything, other than knowing if your friends are using duolingo and teasing them about having passed them.