Just asking since Zed is sitting at a 76% banrate in Brazil, 66% in Korea, 63% in EUW, 63% in EUNE.
I have seriously no idea how such a direct indicator (not to mention the daily forum topics) is consistently ignored.
Cenobite Azaziel3 votesAn assassin with tons of escapes and gap closers that can wave clear like a god? What could go wrong?
AmazingGoat3 votesHe's good at everything but doesn't have as big of an impact late game as some other champions so his win rate in solo queue is meh.
That being said, he's ridiculous during laning phase because he has no weaknesses and can exploit literally any weakness an enemy champ has, and is therefore can be really annoying to play against
Do you have bad wave clear and/or can't take towers fast? Zed can push you to tower with his insane wave clear and either deny you some CS or roam.
Are you immobile? Zed can pressure you in lane and kill you if you try to roam.
Do you like to roam? Zed can follow, or he can take your tower insanely fast.
Zed to me is currently the assassin version of Orianna. He is safe to first pick and can play any match up.
To me a large part of the reason that Zed and other assassins dominate the meta in mid lane so much right now is that it's very hard as an ap mage to punish roams. Following is really dangerous, but it takes forever for most ap mages to take an enemy tower. This lets assassins roam with impunity, as if you follow them they can come kill you, but if you don't they don't have to worry that much about you taking their tower.
Kowe The Ewok1 votesNo big impact lategame? His W leverages his Q, granting him great DPS in addition to his burst. He has the best DPS of all assassins in additition to being one of the safest. Not to mention he has the best pushing power among them.
Terff1 votesWhen he's in lategame though the fact that he scales off AD makes him a godly splitpusher who is too slippery to actually catch and who can assassinate anyone who happens to catch him.
ValyrianBlade2 votesWow... why is NA so bad at banning...
Honestly, I hate leaving Zed up, when first pick I always ban him. He is clearly OP. Not in the "you can't beat him" sense, but in the "he's both safe and can take easy advantage" sense. He has no meaningful weakness apart from being AD (and thus requiring an AP teammate), which isn't really a weakness because he'll take down towers a hell of a lot faster than AP mids do.
Yet in NA he's the 7th most popular ban (2nd everywhere else). Under 30% of games (around 70% elsewhere). Admittedly the top 6 are all okay bans too, but I'd rather see Zed banned over any of them.
Narasimha2 votesIts pretty much because no one can play him to his full potential, and people who CAN play him with that level of skill are usually facing someone who can compete against it.
It was like the Yasuo bans. I don't ban him because I find him a threat. I ban him so my team doesn't decide to grab him.
Subject 51 votesThe issue is the kit that the champion possesses and the kits that were made in the past. I've made a recent post to reply to an issue that is getting to be pretty awful, i'll give you what it said to give you my opinion.
"-If you are going to make overloaded champions, make every champion overloaded as well. Have you ever competed in a marathon moving uphill against an opponent who has a scooter moving downhill, while this scooter has a jet engine and a 9 foot jet fueled fan that blows 95 MPH winds directly at you as you run for the finish? Yeah, it is pretty hard to keep up especially when the referee is like "Uhhhhhhhhh . . . yeah you can't have shoes either, but the other guy can. And those socks that you have, no no no-try these steel spikes for hand. There we go . . .""
Where this was posted: http://boards.na.leagueoflegends.com/en/c/gameplay-balance/3NL0vjdZ-the-pbe-has-been-down-for-2-and-a-half-weeks-first-update-is-a-nerf-to-sona?sort_type=recent
-EDIT-
To answer your question, it would literally kill Zed if he were to recieve a nerf because itemization such as Zhonya for example makes him entirely useless. This item has a 90 second CDR, while twin shadows has 120 seconds. 
has 120 seconds, 
90 seconds . . . 
180 seconds . . . 
90 seconds . . .? 
45 seconds!? Riot has actually managed to punish any form of teamwork for a very long time now, they would prefer flashy and unsafe material for the game to enter rather teamwork and utility champions, if anything you should be complaining about the supports being shot down like they insignificant-most supports are nowadays.
TLDR: Zed is not overpowered. If he is fed, like most fed champions who have gapclosers and uncontested escapes with overloaded kits and itemization that favors only specifc champions such as Zed, they will slaughter.
BeatzBoyFTW1 votesDude, be glad he is one of those assasins that doesn't gets any nerfs. Leave the banrate as it be, assassin champs don't deserve to get any buffs at all.
Kowe The Ewok1 votesWho asked for a buff?????
Grand Eleven1 votesRiot read this post as: more nerfs to Sona are needed.
What have you done ...
L33K Senpai1 votesZed is a slippery one - he's a highly mobile champion with almost no form of CC BUT it is easy to build a team comp around Zed and that is why he is highly banned. In my opinion, he's not good against hardcore CC champions - ahem, ahem; 
stealthfox941 votesPeople are always crying for assassin nerfs.
Kowe The Ewok2 votesThere's a difference between crying and reasonable demands. If you can't see that's there's soemthign off if a character reaches banrates above 70% i don't know what to have to do to convince you... Well, maybe you don't need to be convinced because you already know, but enjoy abusing him?
Anyway. the concept of assassins is difficult to balance, a character designed to blow up others which has significant downsides. Sadly Riots balancing team apparently confused significant downsides with significant benefits - and decided to give a certain champion in addition to his ability to blow up squishies AND tanks near infinite lane sustain and packed three dashes - one including invincibility - into his kit. Plus a spammable AoE slow which can be cast while moving to make him even saver than he already was.