Okay, now that it's been a full patch since Yasuo was released, I thought I'd go in-depth with him and talk about what works well and what doesn't.
This isn't about playstyle or anything relating to how effectively he is used in-game, this is straight by the numbers.
Alright, here's the thing. Yasuo is a melee carry which generally means no-go. Melee carries are rarely viable and if they are, they're generally overpowered. I would say that in his current state, Yasuo is one of the most broken champions in the game for a couple of reasons.
Actually, there's only one reason.
Way of the Wanderer is completely and utterly broken. Double the crit chance at the loss of 10% of the damage he would deal with crits. In other words, with Infinity Edge and Statikk Shiv, you have 90% crit chance. Without IE bonus damage, if you would normally crit for 400 damage you instead deal 360. Versus what would've been 200 damage without a crit. Honestly, if his passive was just this, I'd be okay. Most melee carries have some ridiculously high damage so it's not uncommon to see something like this, especially when Master Yi just straight increases his raw damage by a large percentage and can deal bonus true damage.
My problem is not with the critical strike chance or the damage it deals.
My primary issue with Yasuo is the other half of his passive, Flow.
His flow bar starts at 60 and scales up to 690 at level 18. When this bar is full and Yasuo takes damage from a champion or jungle creep, he gains the 60-690 Flow as a shield for a few seconds. With patch 4.1 Yasuo's ultimate, Last Breath, will now fill his flow bar on activation.
Do you know what else we have for melee carry defensive buffs? Master Yi's Meditate(has to stand still), Fiora's Riposte(only affects basic attacks and modified basic attacks) and her ultimate makes her temporary untargetable, and then we have Tryndamere's heals and ultimate. Most of these have a hard counter. You cast spells on Fiora and group loosely when she ults. Master Yi, use any form of disruption or hard CC/silence and he's screwed. Healing reduction on Tryndamere and his ultimate is more or less the only thing on this list without a really hard counter.
Yasuo gets his shield just by moving around. Flashing, dashing, grabbing Thresh's lantern, anything. The only champion passive that can match Yasuo's shield is Mordekaiser and he has no crowd control and no mobility. Yasuo has both. Malphite would need 6,900 health to match Yasuo's shield and Malphite's shield can't be charged in combat in any way.
Now, already in the EU LCS we've seen Yasuo receive a large number of bans. This wasn't on patch 4.1 where Yasuo gets *buffed*.
If you burn through your shield in a fight and *then* ult someone, you'll receive a boost to your effective health roughly equal to 1,380.
This is genuinely absurd for a champion that can easily his 800+ DPS with only 2 items and boots.
Am I really supposed to believe that it's balanced by it's low duration and medium-high recharge speed outside of his ultimate?
Sad thing is, Yasuo would probably be considered underpowered without his shield.
The primary issue with melee carries is that they have to sacrifice most forms of defence to go for pure damage. On top of having an incredibly good defensive bonus, it's also not always a visible defensive buff.
This passive not only gives Yasuo a large amount of effective health, it also makes it incredibly difficult to burst him down(ignoring the wall that blocks more damaging abilities than anything else minus Poppy or Kayle's ultimate).
Honestly, his wall is the only defensive ability he should be given. It rewards intelligent play and reaction time very, very well while still offers counterplay via melee attacks and non-projectile abilities.
The only counterplay to his passive was to just poke him before starting a fight but with the changes to his ultimate, the only reliable way to counter it is to straight up attack him once and then kite like hell(if you can).
If they can do something to fix the strength of his passive I think he'll be in a great place but until that time I'm pretty sure I'll be banning him more often than not.