Well I have played quite a bit of yasuo, so I guess I'll put my input here; Why not right?
He isn't overpowered truly, but his kit has so many advantages to it. Apart from the (nearly)infinite mobility, one of the best defense spells in the game. He has tons of outplay potential, layered very thickly on his kit. Your kit needs to have equal outplay potential, a point and click non-projectile CC ability, or massive tankiness in order to effectively defeat him. There isn't much you can do to get a good advantage over him is what I am pointing out.
The biggest part I have found when fighting or playing Yasuo, is that you need to be patient. People tend to get really over zealous with him, especially in lanes. If I am laning against Yasuo as say Brand, I normally I have to wait for them to E close to a minion and put himself in tower range. It allows me to get free harass on him. But mainly at any other point in the game, a Brand should never really win against a Yasuo. The applies to most champions I believe, because of Yasuo's kit. Brand as an example, if I go to Q you, use E, if I try to W, you use, if I try to ult, windwall. Great Yasuo player with good decision making will make sure the only ability I have a chance of landing is my E.
Point is, he has lots of outplay potential...
Maybe even too much?