I wouldn't even say Zed's OP, he just lacks any meaningful weakness. What is the weakness of an assassin? Usually it's a craptastic early game, with almost no ability to waveclear unless they use their escape/AOE slows to CS --
. Want to beat an assassin in lane? Punish them for trying to CS, and push your lane (do so safely, of course) so they can't roam as easily without losing tower. I think it's a fair balance. Bad early game in exchange for good late game.
Now, what is the weakness in Zed's kit? I'm honestly not sure. Can he be outplayed? Absolutely. Any champion can. But unless you're drastically outplaying him, you cannot stop Zed from farming and sustaining in lane, and even without any kills, he still has a solid end-game. His Q and his passive make it very difficult to deny CS, and his passive means he need not even waste his Q on minions unless you come within range. The only thing he really lacks is hard CC. What he has: high single-target damage, mobility, an execute passive, energy (spammability), decent AOE with a slow, great waveclear, and the ability to disengage from a dive if he miscalculates.