Now I know what your thinking: "What the hack is something like this is doing on a One Piece Wikia blog?" But do you think I'll talk about another series without adding the One Piece referance? Before you get wrong ideas let me say that I liked both series and I want to do some comparsion but this not going to be 'One Piece should be like Adventure Time' or whatever.
Now you ask what is Adventure Time? To save myself to create a wall of text explaining the series I present you this link: http://adventuretime.wikia.com/wiki/Adventure_Time. Now your probebly asking "What the point?" Well both series are creative and interesting.
Adventure Time takes place in the future, on the Land of Ooo, 1000 years after a nuclear war; in fact Ooo takes place on the post-apocaylptic earth. The entire show takes place on Ooo. The rules in Adventure Time is very loosy. It has colorful cast include a a 1000 years old teenage vampire, an 1000 years old wizard that use to be human, a last surviving human, a multi-colored horse, a princess made out of gum and a dog that can stretch. Magic and science clashes with each other. Adventure Time was full of pop culture and literary referance. The plots are random but one plot was based on an old-fashioned story: A boy and his dog saving a princess from a crazy old wizard. Aventure Time was full of potential. Adventure Time has a love story. It also has many wierd monsters. In terms Adventure Time is whacky.
Now for One Piece, the series takes place on a world that look like it just emarged from a nuclear war but the world of One Piece is a totally different world with it's own history. It employs colorful but bizarre characters. The plot of One Piece is soild and straightforward. The creativity of Adventure Time makes One Piece looks strict in comparision; Oda would not allow magic but he would allow Haki and Devil Fruits to take the center stage. Oda would come up with folklores and tales but they would end up being a supertition. If we see a mythical creature, it could be artifically made by a devil fruit. If we see strange stuff, like a walking skeleton; that means he/she ate a devil fruit. Oda limits his creatity to a devil fruit. His rules compared to Adventure Time is very strict like a devil fruit eaters can't swim. The series relays mostly on scientific facts, which shows that Oda does his research. The series is full of culture and litereay referance and One Piece has a soild potentials.
In conclusion, both series are good and whacky