Frankly, I have no problem with Yasuo by himself. I think he's pretty well-balanced and requires a smidgen of skill to play. He's still a polarizing champ though, like a lot of assassins/slayers, because he can easily snowball into an absolute monster, and easily carry games.
THAT BEING SAID:
What I DO have a problem with, is Yasuo paired with Alistar. This combo is absolutely fuckin' busted. The fact that Alistar has a low-cooldown (relatively; we're not talking an ult here) knock up that he can reliably land on a target of his choosing thanks to his charge, when paired with Yasuo's ultimate, makes it THE most broken mechanical combo in the game. Janna/Yasuo is a close second.
Other knock ups aren't that much of an issue, as they have built-in "out-playability". Rek'sai or Blitz for example have to work to land their knock ups. Janna is another champion that is very nearly broken with Yasuo as well, but her whirlwind can also be dodged (except good luck in the middle of a team fight..).
The reason Alistar is so much worse is because he's a true tank, and goes wherever the hell he pleases. He can single out any target he wishes (a hard-carrying adc, for example), and simply lie in wait for them in the jungle. Late game Yasuo's ult will reliably kill any glasscannon outright, from upto 1200 range away, with absolutely no chance for the "out-playability" that Rito loves to preach as a core design principle.
Dunno what the answer is.. maybe less range on the ult? Either way, this shit needs to be addressed.