The story is one of the strongest parts of One Piece. One Piece is very distinctly separated into it's story arcs, so it would be more accurate to rate the story by the arc, however, all of those arcs would receive (roughly) an 8/10 by me.
I would easily give the story a 9/10 if the pacing were a bit better. I found that in the "Arabasta" arc I was, in all honesty, tempted to skip some episodes, as it seemed to be dragging on forever, but then in the "Thriller Bark" arc I found that the pacing seemed to be unsteadily speeding through the plot.
The animation quality can strongly vary between episodes. When comparing the first season with the eleventh season, you can see huge differences, but considering there was quite literally a decade in between those seasons, I suppose it can only be viewed as heavy improvement.
Some characters look vastly different, from the beginning of the series to where the series is currently, (Usopp, Coby). Which really gives you a feel for how the characters have grown throughout the series, this is something that really stands out to me.
The character's facial expressions have also never failed to amaze me, and certain scenes (e.g. Luffy unleashing a fist into the tenryuubito's face) are mind-blowing.
Now, for the most part, people tend to say the sound doesn't seem too riveting/incredible, though admittedly most of the opening and ending themes didn't seem all too memorable, asides from the remix of "We Are!" which was one of the greatest opening I've ever seen.
However, when I say "Sanji" I can literally hear his theme in the depths of my head, and the music played during major fights is always amazing, and I'll give you the commonly used example of when Crocodile is defeated, and the Czech symphony Dvorak's New World Symphony is playing.
Another great peak of the musical element of One Piece is Sogeking's theme song, which I'm sure not one person who's heard it has forgotten. (LOCK ON!!)
One Piece is quite likely the best example of a massive, diverse, well developed cast of characters. All of One Piece's characters are unique, from their style to their personality and abilities when it comes to battle.
Were you to ask a group of ten people who their favorite One Piece character is you would quite likely get ten different answers, and a good explanation behind it too, since all the characters are so well developed.
The way that the show is centered around the Strawhat Pirates all striving towards their own different goals makes for great character development too.