C'mon guys. Zed isn't THAT strong.
(Be warned that english is not my first language)
Looking at Zed's win rate, it's around 46.5% in solo Q after the 5.2 nerf as a high skill high reward champion. Sounds pretty balanced to me. On the other hand, Ahri has an insane 57.5% win rate after the 5.2 buff (she just got nerfed tho), which made her the most winning champion in the game and I haven't seen many people complain about her. If Zed is as unbeatable as many people say, he should have a huge win rate. But he doesn't. Not even in Diamond or Challenger. It sometimes saddens me how people complain about Zed without knowing how many matches he actually wins. (Zed - http://www.lolking.net/champions/zed®ion=all&map=sr&queue=1x1#statistics Ahri - http://www.lolking.net/champions/ahri®ion=all&map=sr&queue=1x1#statistics)
There are pretty obvious counters to him. Zhonya's. QSS. These items completely delete his ultimate damage. And there are plenty of champions who counter him too - Lissandra, Vladimir, Kayle, Malzahar, etc.
Zed's ultimate has a minimum of 80 seconds cooldown (Without CDR, of course). QSS and Zhonya's both have a 90 second cooldown. That's just 10 seconds different.
Unpokable? While it is true that Zed's harass is good, his cooldowns are fairly high. His Q is evadable like any other normal skill shot. And if it passes through a minion or anything esle, the damage is 40% lower. His W has an 18 second cooldown (Most Zed mains level it up last. Leveled up the most it's 14 seconds). Besides, nothing's stopping you from poking him back.
Ungankable? Yeah, he can escape with his W. But its cooldown is pretty high. What you can do as the midlaner is force him to waste it on you, either by poking or getting close. Then you and the jungler can jump right on him. If you're the jungler, wait until he pops his W for harass or ask the midlaner to bait it. Zed has to use his W for poke in order to have a potental poke lead. So he will most likely use it as often as he can.
Lee Sin and Zed both couple mobility with high damage, but that's what makes them viable. High skill is required on these champions in order to play them at their full potential. Nerfed, they would be useless. Kassadin got slightly nerfed this 5.3 patch, but he was is barely outplayable, while Zed is pretty outplayable.
I think Riot did a good job at nerfing Zed's attack speed. That was a strenght he simply didn't need. It just made it easier for him to finish off people and without much effort. These days, hitting his skillshots is vital for him to kill someone, unless that someone has very low health. Zed does fall somewhat in late game too and his auto attack 1v1s are now considerably weaker. The reason Riot called him a "healthy assassin" is because he can quickly kill people like an assassin should, but requires some skill to do so and is counterable.
Seriously, why would a champion that can't hit 50% win rate after his last nerf deserve another one?
/
No penalty for using abilities? Last I checked energy needs management, Maybe not to the extent of mana but remember there are champs that don't even use resources
etc.