To me a "leader" is someone who drags his side over the line in tough games - think Joel Selwood. Between Northey and now hopefully Roos, we rarely won a game when the other side put the clamps on us. I put that down to too many Whites and Yzes, who regularly went missing when the heat was on, and not enough Blacks (Simon) and Kirks, who just about always played well in big games.
I would suggest James McDonald was a leader (and the league's most underrated player for about 5 years) and Nietz was capable of leaderlike acts despite being isolated at full forward. Not White and Yze, though.