Yasuo.

Because he was the first, viable melee carry Riot has made recently, and he was built with a lot of either new or uniquely-used mechanics.

His passive is two-part; one half is his main means of defense--a shield that has an effective 9-13 second CD and no scaling--and the other half his core damage steroid, done by taking the usual stat converter (example: Vlad's passive AP to HP and HP to AP, Riven's ult total AD to bonus AD), but it's the only one that multiplies Crit Chance. Uniquely, the damage bonus is offset by reducing the total damage of his Crits by 10% (not the same thing as -10% Crit Damage), and this basically makes his DPS similar to Master Yi and Tryndamere at 3 items.

His Q is a cross between a spell and an attack reset; he can cast it in between attacks to get an extra attack, because it applies on-hit effects and crits. It has even higher damage reduction on Crits through his passive to offset the base damage, and it's the only skill (to my recollection) with a CD that scales on a stat other than CDR.

His W is a wall spell, but rather than blocking movement, it blocks enemy projectiles hitting it from either side. This is his special defensive tool, particularly effective against Marksmen and some Mages. It has absolutely no effect against half of the ranged spells in League of Legends, but the fact that it totally negates what it can affect is what rustles most scrubs' jimmies.

His E is a short-range gap closer with no real cooldown. It has a base 0.1-0.5 second cooldown, but it leaves a mark on the target that prevents it from being recast on that one unit for 6-10 seconds. The high mobility provided is offset by the short range and need of multiple targets to be used more than once.

His R is a long-range gap closer that deals a bit of burst and buffs Yasuo, allowing him to ignore 50% of bonus Armor and refilling his Flow meter for his shield passive. It even has a low base cooldown, 80/55/30 seconds. However, it's offset by requiring an Airborne target. Yasuo has only one Knockup, so he either needs to land a skill shot (one that, in my opinion, is simple to avoid) or have a teammate set up the shot for him.

I saw, from his creation, that people were going to complain about him being too strong, despite not actually being as powerful as the forum would have you believe. Right now, the only issue I have with him is the cooldown/duration offset of his ultimate.

People complain because his damage spikes mid-game and gives him a chance to win in the laning phase, even when he was doing poorly. Statikk Shiv makes him shine, and if he builds a quick lead at that point, people never get to the point where they find out he's one of the lowest-DPS melee carries in League of Legends.

Unfortunately, until Riot fixes Crit Chance, they can't really afford to buff Yasuo.

people say he is op when really he just can be played strongly at a high level, but can be countered by a lot of things, mostly high cc. People hate on most mobility champs

Overloaded kit. Double crit chance, even if your crits do slightly less damage, is ridiculously stupid. Potentially unlimited mobility, sure, he has to have enemies around to use it, but that doesn't stop it from being crazy strong. Instead of his abilities costing some kind of resource to maintain, he can spam them whenever he wants and receive a shield instead of, oh I don't know, being unable to cast spells. I'm sorry, but every champion should have some kind of drawback to spamming spells. Rumble is a perfect example of a good unique resource. He powers up if he manages it correctly, but weakens himself temporarily for spamming. What isn't good is champions being able to win lane simply by not having to worry about running out of mana or health in some cases, especially Yasuo since he's supposed to have a bad lane phase. Riven, Garen, Yasuo, and Kat need to be looked at. Energy champs are fine. Rage is a pretty lame resource as well, I'd like it to be done slightly better, but whatever. Another thing is his ult for three reasons. 1. The cooldown is incredibly short. 80/50/30? Are you kidding? Bump that shit up to like 100/80/60. And 2. Why on earth does it give him the extra bonus armor pen?? He's already got autocrits, why make it so he ignores armor without items too? And finally, 3. Does it have to refresh the shield? Maybe fill it up by half, but fully refreshed? A 470 health shield is nothing to sneeze at, and if played correctly, he should be able to refresh it at least two, most of the time three or four times in a fight. (Having it initially, generating by use of E, instant full gain by using ult, and generating by use of E again after ult).

TL;DR His free double crit chance + armor pen, little to no cooldowns, his resource is only helpful and in no way harmful to him, and overloaded ult.

Someone explain to me why people hate him so much?

He's an assassin.

More specifically - he wins by capitalizing on mistakes, generally rewards a 1v1 me brah play-style, is really flashy in what he does, and his main contribution to blowing up a Nexus is killing enemy champions a lot so they aren't going to stop his team. Most champions aren't as good at punishing mistakes as assassins, often function better in team fights by themselves, have a lot of hidden/difficult to see power, and usually don't rely on a strong KDR to force their end-game.

In essence - Yasou is very obvious and very memorable when he's doing well. And he does well against unskilled players.

Same basic reason assassins have been QQed about since LoL went live.

As a Yasuo main myself, the answer is really simple: It is because he is viable against ranged champions.

Having range is a tremendous advantage that most players don't recognize. Typically, often the only options available to a melee character fighting a ranged one is to have a better all in, or some kind of mechanics that reduces the amount of damage received.

Champions that can't do either of these, such as Darius, fail utterly against a ranged champion no matter how good they are against other melees.

In order to make Yasuo viable mid lane without turning entirely him into an assassin, the developers gave Yasuo a plethora of utilities against ranged champions. Unlike most assassins who can rely on their early game damage and spikes from levels, Yasuo is heavily dependent on items. This leaves him with a weak early game. In order for him to survive in lane with this disadvantage, he was given some rather annoying mechanics.

His passive flow shield prevents him from being easily poked down, while his other double crit passive guarantees that he will out scale his opponent eventually, allowing him to fight back.

His "infinite" Blade Sweeps are for retaliating against heavy poke and highly mobile champions such as Ezreal. Without it, Yasuo would have no way of returning damage. His Windwall allows him to escape certain abilities (such as Runic Prison) that would spell death otherwise.

Essentially, his kit negates the "OPness" of ranged attacks. For the first time in League, a ranged champion has no advantage against this melee, and this is something that a lot of people would not expect.

For every advantage a mage such as Lux or Syndra can capitalize, Yasuo has a counter ready for them unlike assassins who would just outright attempt to burst them down. It certainly isn't the normal experience of fighting a melee champion, and it certainly isn't in their favor.

Hence, people don't like Yasuo.

He's just extremely frustrating to play against unless you are playing a certain group of champions.

If you are a mana-using mage who relies on skillshots, then his entire kit is about making you feel useless.

It's very hard to hit skillshots against him because he is so mobile; if for whatever reason he can't manage to dodge a particular skillshot, he can throw up his windwall; if that happens to be on cooldown, he still has a regenerating shield to absorb the actual damage.

So, to hurt him, you need to pop his shield, bait his windwall, and hit a skillshot on a very hard-to-hit champion. To top it off, he can easily build lifesteal and sustain back up what little damage you do manage to put on him. Being manaless along with the above means he's also almost impossible to push out of the lane.

Wind

Too much free stuff in his kit.

Double crit Very strong shield. One that will outscale most supports who didn't build too much AP. Ult is loaded. 50% bonus armor pen, shield, damage, CC, very low CD. Sure it has a requirement to activate. But so does Gnar's. Windwall. Delete's back line for 3-4 seconds in a teamfight. Proper use of this alone can make him OP. Extreme mobility around minions.

While most of this is OK overall. He has almost all of it at level 3 and his first item(shiv) is a huge power spike.

He doesn't need a low ult cooldown and a shield refresh on ulting. You can't even bait the ult and hope to kill him before the animation finishes. I remember playing vs him as Orianna in Dominion. I could poke him down, I could out damage him. But once he got his ulti off I was done for. I even managed to use my own ulti so that the end animation would pull him off me as he ulted. Sadly I found out the hard way you cannot do this. There is very little counterplay to his kit outside of his own skill level.

His windwall doesn't need such a long duration. Make it 0.75s and reduce the CD by half. Make him actually have to defect abilities with it. Not just "oh look the backline, go be useless for 3s while we kill your front line". This would also hurt his early game a bit as he wont be able to hold it up for 3s while he last hits and backs off.

After these two things change, buffs can be looked at. I'm sure he'd need them. Probably gain his bonus flow on windwall ranks back.

To me it balances out, i mean he does hit hard but there are champions on this game who hit just as hard(sometimes harder) with range on him and non gimicky ult. i don't see the big deal about him when i could just as easily get 100-0 with lb with less than 1 full item.