So the boards are no stranger to Yasuo balance threads, and whether or not they're justified is up to the individual to decide, but I think it's important to address why exactly people hate this champion so much.
It because of the anxiety he causes.
No one wants a Yasuo in their game, you don't want to fight one, and you certainly don't want one on your team. The second one is locked in, the atmosphere of the game is liable to change. If he's on your team you place bets on how long until he reaches 0/10, and if he's on the enemy team you wait for him to pop off. Now there are several champions like this: Lee Sin, Riven, Shaco some fourth guy I'm forgetting about, and I'm not gonna say that there's something special about Yasuo, but I think that the fact that just locking him in can be a source of anxiety to both teams is at least part of the reason why so many people dislike him.
Personally I've started having an easier time laning against him, primarily because I play a fuckton of Xerath and Yasuo can't windwall most of Xerath's damage, but my general disdain for him has not gone down any less. No matter how easy of a time I may have against him, the fact remains that I don't want him in my League of Legends games.
This probably isn't new to most people, but I just like the discussion.
TL;DR: Yasuo is one of the carriers of Shaco Syndrome, and the anxiety that causes leads him to be hated.
EDIT: I sort of generalized by saying that this implies to the entire League community, I mainly meant people who already disliked him, or may have previously fallen victim to this stereotype about Yasuo. If this doesn't apply to you feel free to downvote and be on your merry way.