Yasuo will never be balanced until his double crit is removed.
As many have said before, double crit from his passive is the core problem with Yasuo. He scales insanely hard off of just 2 items (Shiv and IE or PD and IE), and still continues to scale after that (Armor Pen, BORK's %HP damage). To make matters worse his Ult basically prevents any counter building (50% Bonus pen for 15 seconds + 35% total pen from LW, around 55% total).
In contrast his laning is not exactly the strongest and those who are inexperienced with him tend to feed because of it. Once he begins building into Shiv or PD it's a slippery slope if you aren't ahead of him by that point. That's not to say he can't be countered (Ryze and Lissandra come to mind, as well as basically any champion with hard CC that is targeted or easy to land). But, what I think is most frustrating about him is he scales incredibly hard and has very low windows of opportunity to fight back (Q has a short cooldown, E can be spammed as long as minions are nearby, lategame he's got a 30 second CD on Ultimate without CDR).
In my opinion he needs a (soft) rework, plain and simple. Remove the double crit and his damage is ruined but he can be balanced from there. In my mind I see him kind of like a hybrid between melee carry and assassin, but with more utility than either one of them. I find it interesting how his Q essentially replaces his basic attack, though it does disappoint me that his Ult's damage isn't very good (most of his Ultimate's power is put into the armor pen). They should also use his flow for more than just "You get a free shield from this".
My suggestions to change him?
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Remove the double crit off his passive, it's no fun for anybody. He reaches lategame status by like 20 minutes and then he can 2 shot anybody when every instance of crowd control is not being used on him. It's mostly a gimmick to boost up his damage anyway, while it also limits buildpaths and doesn't contribute anything to his character archetype.
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Restore the strength of his passive's shields (brings shield/flow values up to 60 - 690 from current values), bring the duration up to 2 seconds. Flow, Shields, and Windwall are what make Yasuo unique and they should definitely be more emphasized in his kit. It fits great with him and his archetype, has counterplay (pop shield and/or Wind Wall before committing to a fight with him), and its what makes Yasuo's laning phase somewhat safe.
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Wind Wall now costs 40% of Yasuo's Flow. In turn, reintroduce the Flow generation passive on his W: "Yasuo is granted 20% of his Flow when striking an enemy champion with Steel Tempest". Lastly, his Wind Wall now has a set amount of HP that scales with his rank in Wind Wall (400/800/1200/1600/2000).
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Sweeping Strike now costs 10% of Yasuo's Flow. Helps to introduce a window of opportunity where you can actually fight back versus Yasuo and puts a limiter on his mobility other than "how many minions are around me?".
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Last Breath - Remove the bonus armor penetration off of this ability. Once again it's not very fun for anybody (just like his double crit) but more importantly it nullified any chance of building defense versus him. Increase the cooldown on this ability as well (up to 100/80/60 seconds?), it's cooldown is way too short for the power it can offer (especially lategame, 30 seconds CD is just stupid). To make up for those two nerfs, increase the damage (200/300/400 + 200% Bonus AD sound about right?) and upon landing, Yasuo's Steel Tempest will be guaranteed to critically strike for 10 seconds.
TL;DR - Remove the uninteractive parts of Yasuo's kit (Crit and free Armor Pen), emphasize his flow mechanics, add more windows of opportunity to fight him.
For those wondering, much of the inspiration of this thread came from this one. http://boards.na.leagueoflegends.com/en/c/gameplay-balance/lBkYPFEW-as-a-plat-1-yasuo-main-with-400-yasuo-games-yasuo-needs-a-damn-rework
*** Update 1: Changed the steroid attached to Ultimate, now causes his Steel Tempest to have guaranteed Crits for 10 seconds (duration might seem high, not sure), though cause of that no base damage increase on Q.**
*Update 2: Wind Wall's HP is no longer influenced by Armor and MR, costs less flow now.
Edit: I see a lot of people saying that I just want to see Yasuo gutted. No. You got it wrong. The intent (heh, Yasuo pun) is that he cannot be fun to play as, to play against, and be balanced at the same time with his doubled crit. He does need some type of powerful steroid to keep him pumping out damage, I get that (which is why it is moved to his Ultimate, and a rather powerful one at that).
Keep in mind, I do see that he's a teamfight oriented melee ADC and wouldn't he still be one with these changes? He still has crits (after he uses Ult, for 10 seconds, full damage with no crit damage reduction, though it is limited to his Steel Tempest), his Ult just has more of a SPLAT feel to it because it has higher scaling (lets be honest he's like a cross between melee adc and assassin, boasting fair burst and high sustained damage).
Giving some type of limiter on his mobility seems fair considering how well he can get around, and it's not that big of a limit either (though, it does cause more interaction than you think. For example, at full flow, your opponent could attack you to prevent you from engaging which would mean you should engage before you have a full flow bar).
If you think these suggestions would ruin him, then share your thoughts on what would suffice to prevent him from not being ruined.