One Piece follows the story of Monkey D. Luffy trying to become the pirate king and going through adventures in order to fulfill this goal.
I find that One Piece works as a great anime for almost any demographic. Children watch, teenagers watch, and I know adults watching too, it's perfect.
The overall plot is simple and easy to undestand, but not easy to achieve which allows for more than one trial, a trial that will always change because of the unique "Devil's Fruits." By not having more than one of any type, the fights can always take a different turn, allowing for a refreshing taste.
The animation has always been great for the time (we can't say it's bad animation if it was made 10 years ago), but there are some thing that could have ben done more clearly. It's little things like size relation. In one shot, a giant may be 20 people tall, but in another shot he's only 10 people tall. Simple things, but they add up, knocking the animation from near perfection.
The sounds and music of this anime are incredible. Not only do they fit the scene, but they personally inspire me. The action music can almost be described as the feeling of pure determination, and the sad music can be heard as more of a lonely music than "sad." This fits the sad backgrounds of all of Luffy's crew as it is apparent that at sometime in their life they felt alone, and the music perfectly illustrates and nails this point into the heads of anyone watching. The music is by far the best part of this anime.
Now we get to the characters. I know characters are supposed to have quirks, but some of them really bother me. It's the repetitive stuff that the eight year olds of Japan get a laugh out of, but it gets tiring. Sanji's constant "Swans" and "Chwans" are nearly at the highest level of my tolerence threshold... The quirks are too crazy, apply to too many characters, and appear FROM those characters too often, it get tiring and makes the show less of what it's about. One of the only things that really saves "Characters" from a lower rating would be the conviction of the characters. While all anime is based around a convicted character, I believe that this is the perfect level of conviction, as it can actually take form in "Haki," allowing for even the less convicted characters to be on the level of convicted characters.
Overall, One Piece rates a 9 from these things when it comes to characters, plot, etc. but definitely rates a 10 on watchability and rewatchability.