Why someone would play 
: Yasuo has no resource used for any abilities except his passive shield; Kayn doesn't have this. Passive doubles crit chance and gives lane sustain through a shield; Kayn doesn't have this. Q deals short-range damage with very short cooldown, can be used to deal damage in a circle when used in tandem with E, and the tornado's knockup has significantly longer duration than Kayn's Darkin form W knockup; Kayn doesn't have this. W creates a wall that blocks all projectiles moving perpendicular to it; Kayn doesn't have this. E dashes directly to a target, and can be potentially used to dash infinitely given a sufficient number of enemies; Kayn doesn't have this. R maintains a given knockup as a suppress, providing additional CC for the team; Kayn doesn't have this.
Why someone would play 
: Zed uses Energy instead of Mana; Kayn doesn't. Passive deals % of max health as magic damage to low HP enemies; Kayn doesn't have this. Q fires a projectile that deals higher damage to the first enemy hit, and doesn't require that Zed stand still for a significant cast time; Kayn doesn't have this. W creates a shadow that mimics Zed's Q and E, and with which he can trade places once—also returns some resource if his and his mimic's abilities both hit a target; Kayn doesn't have this. E instantly deals damage in an area around Zed and his shadow, and his shadow's E slows; Kayn doesn't have this. R applies a mark on the target which deals a portion of the damage dealt by Zed to the target again after a short delay, and Zed can also return to the original cast point at will; Kayn doesn't have this.
Why someone would play 
: Talon's passive applies a heavy damage-over-time effect on enemies struck by spells and then auto-attacked; Kayn doesn't have this. Talon's Q causes him to leap directly to a target, or, if the target is already in range, critically strikes—it also restores health and reduces its cooldown if it kills a target; Kayn doesn't have this. W deals damage in a cone, then deals damage a second time and slows after a short delay; Kayn doesn't have this. E allows him to leap over any number of terrain obstacles with no cost; Kayn's ability to pass through walls has a cost and clearly defined limit outside of a cooldown on already-used terrain. R deals damage in a large circle and causes Talon to go invisible and gain movement speed, and deals the same damage again as the blades converge—if Q is used, the blades converge on the target instead; Kayn doesn't have this.
Why someone would play Kayn: His passive allows him to change forms to become a fighter/bruiser or assassin to match what is needed to combat the enemy team—in his assassin form, it gives him additional magic damage on auto-attacks against enemy champions, and in his Darkin form it gives him spellvamp; the others don't have this. Kayn's Q causes him to dash to the target location (not enemy), dealing damage to all enemies in the path and to all enemies in a circle around the destination—in his Darkin form, it also deals percent health damage to all enemies struck except minions; it could be argued that Yasuo has something similar, but his E only deals damage to one target, and he can dash to any number of targets only on a cooldown on specific enemies. Kayn's W deals damage and slows all enemies in a line in front of him—in his assassin form it has no cast time, instead a delay, and in his Darkin form it applies a knockup rather than a slow; none of these other champions has anything like this—it could be said to be similar to Illaoi's Q, but he doesn't have anything other than himself to use this ability, it deals less damage, and slows more. Kayn's E allows him to walk through terrain with a cost and cooldown, heals him, gives a movement speed increase, and the duration is extended if he's not in combat with enemy champions—in his assassin form he gets more movement speed and immunity to slows; this is very dissimilar to Talon's E in several ways. Kayn's R allows him to "infest" an enemy champion he has damaged in combat (making him untargetable and stay close to the infested champion), and when the ability ends (he can end it early if he wants), it deals instant damage—in his assassin form, it has a longer range and resets his passive magic damage boost on auto-attacks, and in his Darkin form it deals % max health damage and heals him; this is similar to Zed's R only insofar as it makes him untargetable and places him on the enemy champion—it lasts much longer, does not apply a mark for delayed damage, and he cannot return to his original position.
If you actually take a few moments to look at each ability you're comparing, you'll see that not only is he dissimilar to any other champion purely due to a "mixed kit" allegedly from several other champions, but also that he is dissimilar to any other champion in the functions of his abilities. There can be comparisons drawn—they're similar in some ways—but to say that they're the same is ludicrous. You're exaggerating the similarities and ignoring the differences purely so you can complain about the champion. Have you even played with or against him? I've played with and against him on PBE, and he feels nothing like any of those champions you mentioned. It honestly looks like you just skimmed over someone's comparison of the champions and assumed that any comparison means they're too similar, and from there jumped to conclusions without any research or critical thinking. Again, take a few moments to analyze before you start whining that he's a rehash of older champions.
was such a unique champion release as well.
Ult
W
and 
But putting all this unique aspect together makes the champion so interesting.
or 
instead of this corny champion.
R
There are no thing actualy new about his kit, he just a combination of all overload kits of all hypercarries in the game.
