Well, if you take a look at his abilities individually without putting all of his tools together, all of them are pretty simple or lackluster. His Q is basically a skillshot auto, his W is just an ability blocker that provides 0 damage, his E is a low-damage mobility tool, and his ultimate is a situationally-dependent skill.
Looked at separately, his abilities are pretty underwhelming. But when put together...well, the way they flow together is the biggest issue. And when you add in a shield that improves along with his mobility and free stats via his double-crit, it becomes an over-tuned kit.
However, it is also VERY easy to mess up when playing Yasuo, and as a champion, you can clearly see the difference between a player who had played only 10 Yasuo games and a player who has played 100. People will hate me for saying it, but skill IS a factor for him. Low-elo players just do not want to admit when they are out-played.
That being said, having a resource will ultimately ruin the fluidity of his kit, and thus butcher his thematic.