@ all of you rushing to defend this champion...
Open your eyes. Look around the league. There are only a handful of assassins with any amount of real CC in the game, and not a single one has a combination of hard AoE CC and true defense. Let me put this into a little bit of perspective...
Infinity Edge: 25% crit chance increased to 50% on this champion
Statikk Shiv: 20% cirt chance increased to 40% on this champion
With the addition of 9 critical marks, this puts his crit chance at 100% with 2 weapons. This combination of weapons can easily be achieved within 20 minutes, at a point in the game where no assassination targets have any sort of built up defense and where enemy tanks only have a moderate amount of armor and health, both of which are defeated by Yasuo's ultimate.
Offensively, all of his abilities could scale like piss, have extremely short ranges, and still be extremely overpowered. Any ability that grants a huge amount of mobility makes a champion EXTREMELY hard to pin down, even when opponents are incredibly skilled at smartcasting. And while a case could be made for predictive targeting, the fact that he possesses an ability that does not require a specific enemy targeted but can be cast in any direction makes such predictive and intuitive play extremely difficult, especially when one considers the fact that his ultimate grants him a huge armor penetration bonus for what will most likely be the duration of ANY teamfight. This effect basically allows him to continue dealing assassin quality burst damage to heavy tanks and bruisers, negating their usefulness in soaking damage, especially when considering the artillery will most likely be completely unable to punish Yasuo for focusing a tank.
Now, let's examine his defenses. He has a shield which activates on damage and whose shield amount is refilled on rapid movement AND ability useage. Even if that shield only protects from 100 points of damage at max level, his kit and a critzerker build nearly guarantees him multiple shield procs per fight (notice how there is no cooldown in the tooltip) Add to that an INCREDIBLY powerful zoning tool that will shut out nearly 100% of ADC and APC in a teamfight as well as render him virtually unkillable when moving to assassinate a single ranged, projectile based champion.
Finally, let's not forget his CC, of which he has 2! One is an AoE CC that knocks up ALL foes in a line similar to Janna's tornado OR knocks up all foes AROUND him, allowing him to initiate a teamfight AS AN ASSASSIN, much like Malphite would. Add to that a suppression effect to MULTIPLE FOES, an ability that NO OTHER CHAMPION IN THE GAME HAS for the specific reason of how broken it is. Champions like Warwick and Malzahar only get 1 Suppression, and they have to sit there and channel it, making counterplay possible. But if all the enemies that could capitalize on the champion's inability to move are likewise suppressed AND taking damage, and all enemies outside the effect are completely shut out from dealing damage... well, what can anybody do?
This champion is Zed, Anivia, Fiora, Janna, Malphite, Tryndamere, Warwick, and Master Yi all rolled into a single ball of "OMG Riot why the fuck are you doing this to us?"
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