Yasuo's abilities have no cost related to them and their use is limited only by cooldowns. His secondary resource bar tracks his unique mechanic: Flow.
Yasuo can have up to 60 / 65 / 70 / 75 / 80 / 90 / 100 / 110 / 125 / 140 / 160 / 185 / 215 / 250 / 290 / 340 / 400 / 470 Flow. Yasuo generates 1% of his maximum Flow with every 46 units he travels, requiring a total of 4600 units-traveled to obtain maximum Flow. Flow does not decay or generate idly. As Flow generation is based on distance travelled, it is also accumulated while dashing, teleporting or when displaced by crowd control effects.
Way of the Wanderer
Intent: Yasuo's critical strike chance is doubled, but the damage dealt by his critical strikes is reduced by 10% (25% on Steel Tempest).
Resolve: At maximum Flow, the next time Yasuo would take damage from a champion or monster he first converts his Flow into a shield that absorbs damage. After 1.5 seconds, if not already depleted, Yasuo loses all his Flow.
Way of the Wanderer consists of two independent passive abilities.
The damage reduction applies to the total damage dealt (before damage mitigation). A standard critical strike's 200% damage is reduced to 180%, and a critical strike with Infinity Edge deals 225% damage instead of 250%. The reduction is not a subtraction to his critical strike damage statistic.
Critically strike with Steel Tempest does not affect the 20 / 40 / 60 / 80 / 100 base damage (both for the purposes of calculating the bonus damage or being reduced by the 10% penalty).
Once the Flow bar is full, upon taking damage from a champion or neutral monster, resolve first produces a shield that reduces the incoming damage by an amount up to the current maximum amount of Flow for up to 2 seconds.
Damage that is negated by Resolve's shield appears as a blue indicator. For example, -60.
Range: See below
Static Cooldown: 5 / 4.75 / 4.5 / 4.25 / 4
Consecutive successful casts of Steel Tempest within 10 seconds form a chain.
First Active: Yasuo thrusts his sword forward, dealing physical damage to all enemies in a 475-unit line. If cast while using Sweeping Blade, the area of effect is changed to a ~375-radius circle around Yasuo.
Second Active: Same as the first active.
Third Active: Yasuo brandishes his sword causing a whirlwind to tear forward in a ~900-unit line, dealing physical damage and knocking airborne all enemies hit. If cast while using Sweeping Blade, the area of effect is changed to an ~375-radius circle around Yasuo. This active resets the chain.
Steel Tempest can critically strike, having a 25% damage penalty. Without any additional critical damage the bonus damage equals 50% of Yasuo's attack damage. Additionally, on-hit effects will be applied to the first enemy hit within 475-unit range. The cooldown and cast time of Steel Tempest is reduced based on Yasuo's bonus attack speed and unaffected by cooldown reduction.
The cooldown of Steel Tempest is unaffected by cooldown reduction and is instead reduced by 1% for every 1.72% of his bonus attack speed. This is capped at 66% cooldown reduction with 114% bonus attack speed. At max rank, the minimum cooldown is 1.33 seconds (1.4 attack speed).
Yasuo's 3.5% attack speed per level is factored into Steel Tempest's cooldown reduction and cast time reduction. At level 18, Yasuo has a total of 59% bonus attack speed from leveling alone which reduces Steel Tempest's cooldown by 34% (and he requires only 55% attack speed from items, runes and masteries to reach the cap). Steel Tempest's cooldown is effectively reduced by 2.03% per level.
Way of the Wanderer's critical strike damage penalty with Steel Tempest is 25% instead of 10%. With 100% critical damage (base), the bonus damage equates to simply 50% AD (200% × .75); with 150% critical damage (Base + Infinity Edge), the bonus damage equates to 87.5% AD(250% × .75).
Critically striking with Steel Tempest is calculated on cast, not per enemy hit.
The third cast will still knock enemies airborne when it strikes as a point-blank area of effect.
Cooldown: 26 / 24 / 22 / 20 / 18
Passive: Yasuo generates a percentage of his maximum Flow whenever he uses Sweeping Blade in addition to the amount that is generated for the distance moved.
Flow: 3 / 6 / 9 / 12 / 15%
Active: Yasuo creates a gust of wind that travels forward to form a wall 400 units away from himself. The wall slowly drifts forward over 3.75 seconds, blocking all enemy projectiles with the exception of tower hits.
Sweeping Blade will generate ~7% of Yasuo's maximum flow innately as Flow-generation is based on distance traveled. The 4-20% gained from Wind Wall will stack additively (i.e. in addition to).
The gust of wind takes ~0.25 seconds to travel forward from Yasuo to form the wall, but will still block projectiles as with the fully formed wall during that time (for a total of 4 seconds as listed in the in-game tooltip).
The wall grants sight over a small area.
A projectile is any entity with a velocity that is not classified as a unit (champions, minions, monsters, etc).
Moving units are not blocked under any circumstance, including dash abilities. Champion-summoned units, such as Orianna'sCommand: Attack and Maokai'sSapling Toss, are not considered to be units until they hit the ground - and thus will interact with Wind Wall while en route.
The following projectiles pass through Wind Wall (possible bugs):
Yasuo can pass through terrain both before and after he's hit his target - permitting sufficient space for him to make the jump.
Uniquely, all forms of movement-inhibiting crowd control (airborne, snares, stuns, suppression, etc) will interrupt Sweeping Blade when they hit him - rather than waiting for him to complete the dash. This is contrary to all other dashes, which are only interrupted by airborne-type crowd control effects.
The cooldown of Sweeping Blade is unaffected by cooldown reduction.
Cooldown: 80 / 55 / 30
Active: Yasuo blinks to the nearest visible airborne enemy champion to the cursor, instantly generating maximum Flow. Upon arriving, he suspends all airborne units within a 400-radius of his target in the air for 1 second while dealing physical damage to all of them. Once he lands, Yasuo gains 50% penetration to bonus armor for 15 seconds.
Casting Last Breath will reset the chain on Steel Tempest.
Last Breath is an auto-targeted ability that can only be activated if there is an airborne enemy champion nearby. The cursor's current position will influence the ability's auto-targeting, although it is not technically using Quick cast and having your cursor near a valid target is not required to activate the ability.
Bonus armor refers to all sources of armor beyond the target's base armor and armor per level. Bonus armor refers specifically to armor granted through: runes, masteries, items, auras/buffs and champion abilities.
The bonus armor penetration applies before percentage armor penetration.
Using Last Whisper as an example: "Subtract half the enemy's bonus armor, then subtract 35% of the total remaining armor." 
Yasuo may cast Steel Tempest while casting Last Breath.
Note that "Airborne" is a classification of enemy enforced crowd control and not a visual description. Only airborne-type crowd control sourced from an enemy champion will count for the purposes of Last Breath.
Jumps, including abilities recognized as self-CC, do not classify the unit as being Airborne.
Bouncing animations, such as Zac during Let's Bounce!, do not classify the unit as being Airborne (though the enemy targets he hits with it are knocked Airborne and will function with Last Breath).