[color=red][edit by mod,Spoliers start 3rd paragraph down BEWARE] From the art to the story and the characters everything seems larger than life in this manga. And that's a great feeling! The author seems to have made it a general theme in this work where everything has been amplified and exaggerated to gigantic proportions. The artwork, while seemingly simple at first has many smaller details that other manga seem to leave out; for example the little crease next to a character's lips as he smirks gives it a little bit more life. The characters are all extremes of one kind or another and rarely strike a balance outside of major plot points. Even the locales are amazing and ridiculous which give it such a unique charm. Many critics of the series complain that it's too 'generic'.
"The story is generic at best. I don't see what's original about [insert shonen hero of a certain character class] wants to find [insert legendary item] so he can be best of [insert character class]."
Like Naruto, the general theme behind the story seems generic the way he gets there isn't. You can complain about originality but it's like the blinders on a horse, if you stop there you'll never see what's so great about the story, the characters, the setting and the grandeur of it all.
Another unfounded complaint I've seen is this;
"Love the 1st few volumes but good lord..has any character matured beyond Usopp's desire to have the Merry Go not be destroyed and Luffy developing Gears (ala DBZ powerups?)? The answer is no..this is a vry long very very similar manga from whence it began almost 10 years ago."
Hmm... let me think. Nami isn't just concerned about loot and treasure anymore as she has friends and family that she wants to protect. Sanji instead of working in the floating restaurant has decided to travel the world and see what kinds of food there is as well as developing strong relations with the entire crew. The characters did change from when they were first introduced, did they do a dramatic 180? No! The basis for good stories are characters rooted in a certain mindset that drives them to do things. Luffy's caring about his crew shows a deeper and more serious side of him. Another example from another manga would be Sasuke from Naruto. His primary drive is to defeat his brother where that drove him to betray his friends there is no impetus for this to happen in One Piece.
All in all a great manga with interesting characters, an amusing plot and an amazing, and consistent shock, theme.