What made Zidane so special was that he was incredibly well rounded as a player. He was technically and physically dominant at the same time. I remember Roy Keane and Paul Scholes praising Zidane for his grit and physicality. Zidane had more tools in his locker than the likes of Iniesta and Modric. Apart from his obvious technical superiority, Zidane was great with his left foot, strong in the air, adept at taking set pieces, had magical tricks and flicks and was a leader. Iniesta and Modric don't really have these qualities although the latter has shown leadership qualities for his National Team.
You can't quantify this but Zidane also had a special aura and presence on the pitch. He was the main man unlike Iniesta and Modric. While tricks and flicks aren't the most important thing, players like Zidane and Ronaldinho will always be loved for the extra entertainment factor. The things Zidane could do with a ball were crazy. Most players and managers are still in awe of him.
Why do you think he's overrated? Inconsistent performances in league campaigns?