This is a review from a Chinese boy so it may be hard to understand what I want to express because of my poor English. Anyway, let's start:
story：as we can see, the story part of op is exactly what makes it unique and amazing. The structure may be simple: just luffy with his crew going to different islands and helping others coincidently or not. But to be precise, every arc is full of good plot and amazing events. And even the backstories are hitting!
art: oda's art may not be the best,but his art is definitely unique and impressive, the buildings in op are always splendid and attractive, and oda is always trying his best to draw every chapter. Even though some female characters' face may look the same, but most time it is acceptable to me.
character:I think it's easy to find out oda put much effort on the characters, even a unimportant role may give you some chills and feelings.
enjoyment: for myself, it's the best reading experience with so much fun and other mixture of feelings. every arc I am impressed by some themes oda want to show us .
overall: a masterpiece without doubt. Personally, there are not many series can "impress" me from some aspects, but one piece does impress me a lot.