With the claim that his AA animation looks boring by comparison, one must know that Katanas are not durable weapons by default and requires the user himself to be precise to bring out its razor cutting potential. That's why Yasuo's attack animation is very straightforward, he just draws, do one or maybe a few cuts then sheath and repeat. However it's intentionally designed that way, as it takes the assumption that Yasuo only strikes at a sure opening, that's also why he has a double crit chance by default.
Sure it may not be flashy and all, however theme wise, I think it pretty much hits the bulls eye.
As for the particle effects, I think Yasuo's isn't bad really. It just isn't super flashy.
Also just a side info,
Yasuo's AA animation changes from 1 handed to 2 handed between Classic and Project. As Riot was only aware that katanas are meant to be used 2 handed after the blade is drawn (with only a few exceptions in techniques and school) when Yasuo had already been released. Details like this is always good to know.