Although I think an XP leaderboard might be nice, at the same time I worry about how people might dispute the legitimacy of levels. Some people have left their computers running nonstop for XP without actively playing games, and although I'm indifferent, other people might not think that that's fair.
Leaderboards aren't super high on the priority list at the moment (some neat features are in the works), but I'll make sure I don't forget about them :)