Yasuo's Wind-wall is balanced, but its execution is what breaks him.
I'm just going to get straight to the point, here. A wall that breaks projectiles seems perfectly fine to me. He's a melee slayer guy, and if he's jumping feet first into danger, there's nothing wrong with a tiny bit of safety. But there are two main issues that come with it that can be changed.
#Firstly, his wind-wall is thicc. I don't mean it's long, but actually thick. It's wide enough to stand in, which he makes people do. He can put his wall on top of the enemy ADC and use it as a literal Teemo-esque blind until the ADC walks out of it. it's kind of like Grave's smokescreen, but without the range, slow, or debilitating nearsightedness it causes. So if you're trying to engage on him, he can run up to you really close, lay it down, and get some free harass on you, all while you're forced to run away to even manage to break his shield. If a Caitlyn, Ashe, Jinx, Miss Fortune, etc, is trying to ult him, he deserves to be able to block it with a well-timed wall. But if a ranged champion is simply trying to use an auto attack, putting a wall right on top of them to banish them from using any attacks or skill-shots is just cruel. INVERSELY, once the ADC walks away from the wall, Yasuo himself can stand inside his windwall, attacking freely whilst negating attacks from all sides. Like, ew. #Solution: Make the wind-wall thinner. So Yasuo can't abuse its massive hitbox.
#Secondly, when Yasuo casts the windwall, it shoots out from within him as a cone. Why is this bad? Well, it turns his mere "Wind-wall that blocks abilities" into a "Free spell-shield that then turns into a wind-wall that blocks abilities." You've seen YouTube videos where a Yasuo blocks a Thresh hook by casting his wind-wall sideways, haven't you? It's because of how his wall is designed. Since it casts from within him, any skillshot that's almost about to hit Yasuo will get negated, as if it did hit him, but Yasuo just used a cleanse to physically push the projectile away from him. It's too good, and it's too safe. #Solution: Make Yasuo cast his wind-wall from left to right, in the way Victor's E does. Give it the same length, distance and parameters that his current wind-wall has, but change its summoning from coning out from him to coming in from a side. You can pick the side; I don't care. Probably from left to right, because that's how he would naturally cast it when unsheathing his blade. Aesthetically speaking, of course.
Sooo, what do you y'all think?
exists, as do other champions who would abuse this paradigm to render windwall useless.
vs 
breaks it with 2/3 of her W or 1/3 of her ult.
can break it down with E spam.
2/5 of his W rockets would break it.
1/3 of her Q missiles would break it.
her ult would break it in under half a second.
2/3 of her Q would break it, if there are that many targets on the other side of the wall.
can just throw his Q and E at the wall, which takes no time at all and doesn't put much of a damper on his dps considering all of it comes from autos.
2/3 of 1/3 of his E would break it.
if his Q counts, he would break it nearly instantly. If it doesn't, he can double cast his E for a similar break speed.
oh no, one of his fifty "E-Q" combos in that teamfight got blocked. The horror.
if both Q targets are behind a wall, the wall breaks.
2/5 of his Q would break it.
the first two balls of her ult would break it.
2/5 of her Q would break it.
two bolts of his ult would break it.
suffers greatly vs Yas due to his Q being very important to proc his passive early. So the windwall is a quite annoying, but it can be dealt with. second I don't see why this is a big issue. if these are champs that the windwall is ineffective against its not really that big of a deal. first its not 1/5th its closer to 1/7 if there is 140 champs I am not sure on the total number. second that would also me 6/7 of champs its very effective against. what does it matter that it is ineffective vs these champs as the Windwall is just like a bonus skill that he doesnt really need to win especially vs Melee. lets say for some reason you went against 5 of these champs in game. Do you know how much easier it would be to get a 5 man ult off since they are all melee and have no real poke. The windwall is more of a cherry on top vs ranged then a needed ability for Yasuo.
{
first item, potentially 
later, there is just way too much damage yasuo denies his opponent from returning back to him while he deals an excessive amount himself.