Yasuo is what we like to call cancer. He's an OP champion with double critical strike chance of a normal champ, a shield, a projectile blocking wind wall, a 0.5 cd dash that stacks two times on damage, and a Q that on the third use allows a whirlwind applying melee on-hit effects to people over 6 body lengths away, that's not even mentioning the fact he has an ult that has 29 seconds of CD on level 3 and grants 50% bonus armor pen for 15 seconds after use. People actually think this is a well balanced champ, and a nice history of their matches usually shows they play him, which is why they want nothing to happen to the guy.
However, he has some major counters. Malzahar, Darius, Garen, all of them work extremely well with holding him down. Sion actually is pretty good at taking him down a few pegs, build thornmail and frozen heart and then focus on health and health regen. Don't bother trying to hit a knock-up Q unless he has no minions to E through, and slow him with E and possibly frozen mallet's passive long enough to get a few good shots in. Basically, don't stop at armor, focus on health and health regen!
The best counter, of course, is simply for that champ to lose the double crit and bonus armor pen on ult, he's fine without those two things, but oh well.