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.