His kit has a huge number of different abilities whether it be passive or active, he has no resource system at all, tons of mobility, a non-ultimate that counters probably half or more of the abilities in the game, high base damages, low cooldowns, a free shield, doubled crit chance, easy harass pressure with his tornado, so on and so forth.
Even so, I'd hesitate to call him OP.
It's just that while yes, he's high skill CEILING, he's also LOW skill FLOOR. A good Yasuo player is incredible. A bad Yasuo player in a low level tier of play can just mash keys and achieve success more or less due to how versatile and loaded his kit is.