Heh, if you're comparing One Piece's art quality to other anime's, Persona 4 probably shouldn't be your first choice.
A lot of those series have higher budgets because they are only twelve (sometimes 24) episodes long, not 550+, and they make a killing off of otaku DVD/Blu-Ray sales. A fairer comparison would be to a fellow long-running shonen like Naruto, Beelzebub (though that may have ended already, not sure), Fairy Tail etc. And they all tend to look like crap compared to the shorter, seasonal shows (Sket Dance might be an exception though, of the 40+ episodes I watched, all of them looked really good).
The Naruto anime blows the One Piece anime out of the water in terms of animation, pacing.
The animation that they had in the recent flashback before to when Naruto was born was amazing. I wish Studio Pierrot was the one doing the One Piece anime not shitty Toei since they actually know how to pace an anime properley and give important moments the animation it deserves.
Studio Pierrot>>>>>>Toei when it comes to Weekly animes.