I'm just gonna shamelessly plug my runes, masteries and typical builds here and discuss why I think they're good.
Reason 1 stays the same, but
I have four different mastery pages, each with their own uses. I'll show the two I use most, followed by two others that are also pretty good.
Why 12 in Resolve?
Why Runic Armor?
This increases the power of your Regen and Lifesteal, which is huge both early and late.
If you play right, you shouldn't need that bonus 45HP from Veteran's Scars.
+50% Base Health Regen is great for keeping yourself in lane.
It increases to +200% Base Health Regen when you're low on HP, which not only helps you even further to stay in lane but it also can save your life. I value survivability highly.
Why Expose Weakness?
Yasuo can easily stack Fervor up to max due to being a melee champion.
Since Yasuo is an AA-reliant champ for the most part to deal his damage, Fervor scales well and gives him a lot of extra damage.
This mastery is better lategame but is also decent early on. If you don't need the early game power of Grasp this is a great mastery to take.
Increased LS is amazing on a tankier Yasuo build, where you may not build a LS item.
Since this works on minions and without crit, this offers you great survivability early on.
NOTE: I will only use this with a Triforce build, where I may not get LS. I wouldn't use it in a build where I get lifesteal (I would use Fervor).
The MS helps you get back to lane quicker, as well as roam a bit faster.
Yasuo does not need any help farming.
The early gold is very good. You get items more quickly and, even if you miss CS you still get one gold which, combined with harass, really will add up.
Dangerous Game is also fine, but I feel Bandit is better early on, which is what this page is all about.
Why don't I use this page?
NOTE: This page is still excellent against squishies both mid and top, but I just prefer Fervor in most cases (except against Riven).
Swiftness grants 15% Tenacity and Slow Resist, which is incredible on a melee champion like Yasuo who is hurt badly by CC.
Yasuo is generally not built to be the tank. You want the Tenacity over increased resistances in most cases to keep you alive and putting out damage.
Here I'll just post my two typical builds. NOTE: I always build 2 defensive items, because the survivability allows me to stay alive and continue dealing damage. You don't HAVE to build this way, and I will post some other good items after this section.
Shiv or PD?
Shiv is good if you need the waveclear or the hybrid damage.
PD is good if you need the dueling power, also has more AS than Shiv.
CDR lategame makes you good in teamfights, allowing you to ult and windwall more often.
HP and MR are great against mages.
Increases the power of your regen and lifesteal, meaning you gain even more survivability.
Offers protection against burst.
Increases the power of your Spellblade procs (since it increases your base AD) if going with a Triforce build (yes, this is in the IE part of the builds because it's also useful there).
Why Dead Man's Plate?
MS is great on Yasuo, letting you charge your flow even faster as well as catch up to enemies.
Gives a little extra damage just for moving, which Yasuo does a lot of.
Reduces the damage of Crits, which is good against the likes of Tryndamere (or several ADCs).
Active gives you great sticking power.
Why Trinity Force?
You lose a bit of damage and exchange it for utility. Trinity Force gives you:
These things make Trinity Force great for skirmishing and split pushing.
Why Titanic Hydra?
Why Frozen Mallet?
Why is Lifesteal Situational?
Shield protects you from Magic Damage, making you more resistant to burst.
Gives AD, MR and flat armor penetration, which are all good on Yasuo.
When the shield is popped, you gain 25% AS and 10% LS, which are both incredible stats on Yasuo.
Why Blade of the Ruined King?
AS, AD and LS are all amazing on Yasuo, even after the nerf to BotRK.
The active gives you some good chase/sticking power as well as an escape.
The passive makes you deal a lot of damage to health-stacking tanks, which is very useful for Yasuo.
If you get this in an IE build you don't have to get Greaves. Yay(?)
Mercurial now offers a lot of LS, making it very strong on an AA-focused champion like Yasuo.
QSS active allows you to escape their hard CC, which can mean the difference between life and death.
Why Mortal Reminder?
Why Lord Dominik's Regards?
They have so much armor that even your ultimate isn't enough to get through it all.
You need something to help you take down Health-stacking champions (though I would prefer to get Blade of the Ruined King in that case).
There are other items, runes and masteries one can take. These are just the things I use most often. If you don't like what I offer, feel free to check out what other people might have to say and try their preferences out!
EDIT 1/30/16 - Cleaned up.
EDIT 2/03/16 - Changed TF build (cleanup), Changed IE build (cleanup), moved Fervor page up and explained it a bit more, cleaned up Warlord's explanation, added a little more explanation in the "Other Good Items" section.
EDIT 2/18/16 - Cleaned up Masteries, Changed TF build, Changed IE build, Changed Runes, added notes to Grasp and Thunderlord's information.
EDIT 2/23/16 - Edited an error in the Greaves description due to rune changes.
EDIT 2/25/16 - Edited Masteries slightly.
EDIT 3/01/16 - Edited Trinity Force build slightly.
EDIT 3/16/16 - Edited Trinity Force build slightly.
EDIT 3/18/16 - Clarity edit.