The problem with champions like Zed isn't innate to them, it's the state of runes and items.

Zed is not a bad design. In fact, he's one of the best designs to ever be put in this game.

He has a design that makes one key assumption; that Zed needs his ultimate to score a kill, that without it he's only going to deal about 2/3 of his target's HP.

Back in season 3, this was a safe assumption.

The best rush item was Blade of the Ruined King, because its active damage/slow upped Death Mark's damage quite a bit. The relatively low AD on the item meant that this left his abilities fairly weak. Now, he just throws on Electrocute, rushes Duskblade and oneshots you.

Similar problems exist for some other champions.

Lee, for instance, was designed around the assumption that the best build for him would remain the same. For those who don't remember, back in the day Lee would go 1-2 damage items, often detouring for sightstone/wards (delaying damage further to keep him in check) and then go tank. Now, he can literally fill his inventory with Lethality items and play the same assassin playstyle he does in the early game forever.