Zed is the Lee Sin of midlane.
Is he, in and of himself, broken? No, not at all. He, as a champion, is balanced (mostly.)
However, what he does to the game is HORRIBLY FUCKING BROKEN.
He forces mid lane to play around his existence. Just by being in the champion pool, he shapes the mid lane metagame.
Same with Lee Sin. Lee himself isn't broken, but he completely shapes the jungle metagame. He forces every jungler to be able to compete with him, even if he's not in the game. He sets the bar.
It's a problem, because it's basically safe, reliable, strong kits being so safe, reliable, and strong that it sets a minimum 'bar of viability'. Champions below this bar are basically unplayable, while champions significantly above this bar, or at this bar, are often considered OP (Elise, Fizz, etc. Pick a champ that's bitched about because they're strong, and this is why.)
It's a lose lose situation: Nerfing the champion to remove the 'bar' is unfair to everyone who plays that champion, and it's not healthy. Buffing every other champion to that 'bar' or above it is equally unfair in the opposite direction. Reworking the champion, see previous.
Riot fucked themselves in the ass when they let Lee Sin and Zed dominate the metagame for too long. Zed and Lee are literally never going to be unviable, even if they're not the strongest at their role or even particularly OP: Their mere existence demands that the champion pool play around them. It's retarded.