So, Sona, at some point a million nerfs ago, was overpowered as hell, and still received nerfs because she still was overpowered? The same champion whose heal at level 1 is less than that of a single Relic Shield charge? Same one whose only CC is her ult, and barely does damage unless you somehow stun them under tower? Because a champion that has, for some reason, received huge amounts of nerfs = OP?
What part about Kassadin, honestly, is toxic? Is it his ult? 
all have much more mana/energy efficient gap-closers that can potentially be just as powerful as Kass's mobility, except without the scaling costs. After the nerfs, almost all of those dashes and blinks and what-not will completely outrange Kassadin for a fraction of the cost with non-ultimate abilities. Q Silence was removed for a channel interrupt and a very short-lived magic shield. His W is tied to his melee, short-range basic attack, and is about the only thing in his kit that does damage anymore. His E requires 6 spells to activate, and all it provides is slightly more damage than his Q along with a slow. Again, all of that is very short range, meaning you have to ult into range to use any of it. If you reduce his ult range, keep the scaling mana cost, and keep his short-range abilities, he will be absolutely useless
You're right that he has a high ban rate (~40% ban rate in all ranked games, ranked #4 of all champions behind Nidalee, Jarvan and Irelia [in EUW, he's 3rd top ban behind Nid and Irelia at nearly 55% of ranked games. In Brazil, he's #6 at around 30%]). However, he is not "pick or ban", he's not even in the top 15 most popular for any region. In all ranked games, he has a 9.38% pickrate and 47% winrate (if you have both a low pick and win rate, then generally there might be something wrong. Notice I said generally, as win/pick rate isn't a good measure of champion strength unless it's some insane outlier). He barely sees more play than Malphite, but at a much lower winrate.
Sources: http://www.lolking.net/champions/ and http://www.lolking.net/charts
There, I used data as well. Shall we use more?
Kassadin passive: -15% magic damage taken and ignores unit collision
Zed passive: Zed's basic attacks do 6/8/10% max health to target as magic damage when target is below 50% health. Effect only occurs once every 10 seconds
Kass Q: lock on, deals 70/95/120/145/170 (70% AP) and gives magic shield that lasts 1.5 seconds at 40/70/100/130/160 (30% AP). Mana cost is 70/75/80/85/90 on a 9 second cooldown at 650 range
Zed Q: skillshot, deals 75/115/155/195/235 (100% AD). Second target receives 60% damage. Shadow Shuriken deals 50% damage and restores energy if both shurikens hit the same target. Costs 75/70/65/60/55 energy on a 6 second cooldown at 900 range [little note, a rank 5 shuriken with no CDR is fully refunded after cooldown expires because of Zed's energy regeneration]
Kass W: basic attack enhancer, passively causes his basic attacks to do 20 + 10% AP damage on hit. Active causes next basic attack to deal 40/65/90/115/140 (60% AP) and restore 4/5/6/7/8% missing mana on hit (x5 mana restored on champion hit). On a 9 second cooldown with no cost
Zed W: linear skillshot, passively increases Zed's bonus AD by 5/10/15/20/25%. Active creates a shadow for 4 seconds that mimics Zed's abilities and can be swapped with. Any hit from the shadow's abilities restores 20/25/30/35/40 energy. It is on a 18/17/16/15/14 second cooldown with energy at 40/35/30/25/20
Kass E: conic AoE, passively gains charges whenever a spell is cast by or around him. At 6 charges, he can activate it to do a 50/60/70/80/90% slow for 1 second and do 80/105/130/155/180 (70% AP) damage. 700 range, 80 mana on 6 second cooldown [another note, it's gated by the fact that 6 spells have to be cast around him, so cooldown isn't always relevant)
Zed E: instant AoE, does 60/90/120/150/180 (80% AD) and slows for 20/25/30/35/40% to all targets within 290 range. Each champion hit reduces cooldown of his W by 2 seconds. Shadow Slice also slows, and provides 50% of the current slow of his E on top of Zed's original slow (20% becomes a 30% slow, ect.) but it deals no additional damage. This also restores energy. Cost is 50 energy on a 4 second cooldown
Kass ult: ground targeted blink, can teleport Kass 700 units (soon to be 450 units) away, and deals 80/100/120 (2% max mana) damage to targets 150 units from target location. Each subsequent teleport within 20 seconds adds a stack, which doubles the mana cost and increases damage by 50%, up to 4 times. Initial cost at all levels is 75. After riftwalking n times within 20 seconds, the total cost over time is 75/225/525/1125/2325, dealing a maximum of 240/300/360 (6% max mana) on the final stack [previous damage I listed is based on level, not how many stacks you acquire]. Has a 7/5/3 second cooldown
Zed ult: single-targetted dash, Zed becomes untargetable for 0.75 seconds and dashes behind target. Marks the target and creates another shadow in the cast location that lasts for 6 seconds that functions the same as his W. This shadow can also be swapped with. After 3 seconds, death mark triggers, dealing 100% of Zed's AD (+25/25/50% of all physical and magic damage dealt to the target in those 3 seconds). Has a 120/100/80 second cooldown.
So, which one do you think does more damage? Which one has the higher pick and win rate? Which one does both physical and magic damage, has insanely low costs with insanely high regeneration, and does extremely massive damage in 3 seconds of time?
"I'm a shit player and i can't beat assassins in lane"
Does Kass have the waveclear Zed does? Does he have both the engage and disengage? Does he have the flat-out irrelevant energy costs on all of his abilities? Please, provide a stronger argument other than I am somehow abusing the power of the circle-jerk (which I don't think it's a circle-jerk, I think it's a bunch of people that have realized how powerful Zed truly is)
?
then I think there is a slight problem, don't you think??
