Why would he build %Health on hit damage when he has no attack speed steroid?
...because his Q procs on-hit effects, his cooldowns scale with attack speed, and it has incredible synergy with his ultimate? BotRK is core on Yasuo, and Shiv->BotRK->IE has been his standard build for months, now.
Why would he not build Critical Chance or Attack Speed instead when he has the ability to gain CDR from Atkspd and double crit chance?
Because he has six item slots, and only needs two of them to get guaranteed crits.
If he's not intended to ult backline champions why does his ult have extreme range?
He's intended to ult anyone and everyone. Target agnostic, as I said.
If he's meant to be a person fighting low burst high tankiness enemies why give him an anti-burst defense rather than inbuilt sustain?
Because he needs to be able to survive mid lane, which is full of high burst low sustained damage mages and assassins.
Regarding his lack of sustain, I'll defer to CertainlyT -
"I see this posted a lot. Mana is only one way to gate a champion.
In the case of Yasuo, the resource gate on spells is actually inverted. Yasuo is gated by the NEED to act. If the Yasuo player is not successfully casting Q and E, he's falling behind -- Q builds stacks toward his knockup. E builds his passive. Each of them also leaves Yasuo vulnerable. In lane, Q starts at a 0.42 second cast time and E alerts the enemy to his exact position in 0.6 seconds (the time of the dash). Using those momentary losses of control to your advantage as the opponent can help you land skillshots or reposition against him. In team fights, the need to build Q stacks requires Yasuo to find targets (like the Wraiths, minions, or enemy champions that move into his team's lines) before he can cast his more powerful Q3. The E requires him to move around in a fight to keep his passive working, making him less of a target chooser and more of a target taker.
He also has no inherent sustain, rare for a melee, and most of the time is short range, allowing him to be harassed. We could have given Yasuo mana, bumped his durability/regen up a ton, and given him long range waveclear tools, but instead we made him manaless, fragile, and short ranged. Personally, I think this was the right choice as health as a resource is something that the enemy actually has control over. Mana regen is mostly a function of itemization. It should be more satisfying to maintain proper range while harassing Yasuo out of lane than to build enough armor that Pantheon cannot kill you with spears before OOMing.
(http://www.surrenderat20.net/2014/01/red-post-collection-certainlyt-on-yasuo.html)
