It's not his wind wall, damage, or knockup that actually annoys people. It's his shields (They just don't know it because they don't see it in dynamic play).
His shield has a perfect Synergy with 
which uniquely allows him to always either be charging his passive shield (which has no cooldown, charges faster in champion encounters, and scales from 100 damage to 510 damage shield) or his BT shield while his Passive shield is active.
And since 
drastically reduces his damage received, he generally is hard to chip through on those shields.
And he can add 2-3 more layers of shields with Nullifying Orb, 
(Which also heals him with Life steal), and I've even seen a Yasuo take 
.
His shields are so powerful that he can literally stand in front of a Baron, Point Blank, and just spam Q to kill it, and not take a SINGLE point of damage, because his Shields are constantly soaking it all. And vs a Baron, Yasuo's damage is only about 200 per hit, so he's receiving less life steal/damage to convert to shields than he is versus champions, which he does 3 times as much damage to.
^ Should give you an idea of how significant the shields actually are to a fight. (You just don't see it in a fight as much because you are constantly chipping at Yasuo's barrier which makes it appear tiny; But Qing a group of 3-4 minions gives him about 600 points of shielding). I've actually tried something vs a 
: I literally stood in her Ult, and took 0 damage while I killed her, just Qing away. Didn't even have to wind wall or anything.
If Riot nerfed 's shield or reduced Yasuo's own scaling (to say 100-310), You'd see him being a lot less frustrating and more about actual skill. His counters would stay counters the entire game, rather than getting butchered post level ~ 11.
P.S. The reason Yasuo generally has a low win rate is because people play him like Talon: The incessantly try to 1v5 over and over, when in reality their champ is godly in a team fight. When you 1v5, You're always giving 1 death and 4 assists to the opponent, even if you do kill 1-2 people. Over the course of the game, that adds up, and is why many Talons/Yasuo's lose games... because they're constantly feeding entire teams gold enough to have power to take opportunities and push turrets and end games. If they played him like he had a risk of dying, and worked more as a team, he'd go up 10% and rival 
as one of the most broken stats champs in game.
