The majot example of his leadership is when he just said "we are going to stop the Birdcage" and the others went to help him (and he would have failed without Fujitora, an admiral).
Uhh that’s exactly what charisma is. The ability to inspire others to follow you. If Random Gladiator #69 ran up to birdcage and suddenly started pushing, would anyone follow his example? Everyone thought that attempting to stop birdcage was impossible, but Zoro’s strength, conviction and leadership by example convinced them to follow him.
A couple of situation in which he has given good advises to Luffy doesn't make him a leader: he can be noticed as a strong one or someone with the aura of a feral beast but he is not the charismatic one in the crew. That is Luffy.
Except that it does. A leader is also someone that remains cool headed and logical while others are losing their minds so that he or she can determine their next course of action. And what do you know? Others on the crew admit that Zoro’s advice is logical and end up doing as he says anyway! That’s the very definition of leadership, clearly no one else had a better idea.
Now, what else do Zoro and Luffy have in common? That’s right, care for the next generation and the ability to inspire them. Case in point, Momonosuke and Chopper looking up to Zoro as an older brother figure and the children of Shimotsuki village all trying to emulate Zoro. Tama looked up to Ace and Luffy, both COC wielders who promised to make her life better, just as Momonosuke admires Zoro’s warrior spirit and listened to his advice while training. Now, which other characters have shown a great emphasis on the next generation? Rayleigh, Whitebeard, Shanks... what could these people possibly have in common?