1. The characters, they are so wild and wacky but they have this never give up attitude that shows up when necessary. They actually look like they have fun, and some seem badass at first, but are in reality dumbasses.
2. The story is interesting and it revolves around an adventure more or less. I'm content in seeing the crew move from island to island, its interesting either way. There are many different subplots that are intertwined as well, such as Robin's Blank Century thing with the Poneglyphs(forgot how to spell) and how Gol D. Rogers could read them as well, compared to how Luffy is trying to become King of the Pirates who used to be the currently deceased Gol D. Rogers.
3. The Villains aren't really villains in some cases. They make you laugh and generally have more fun personalities. Especially Buggy he's awesome.
4. Although it has this fun nature and light atmosphere, One Piece can drop to an exciting tone or a dark tone.
Spoiler (highlight to view)
One in particular would be when Usopp gets all angsty and sad for Going Merry, angry at Luffy for deciding to put her down although he knew Going Merry couldn't make it any longer. So in a very emotional battle, Usopp had a fight with Luffy.(Which he lost)
5. Every Character has some background in the crew. One Piece went through each of their pasts which later revealed their goals and the reasons why they are in the circumstances which they were in when they all first met Luffy and/or the crew.
6. Everything is somehow connected in One Piece. There is always something that bridges a connection between characters, may it be through foes or friends. It just makes the One Piece universe that much bigger.
7. The several mysteries that are in One Piece such as the "Will of D" or the poneglyphs & the Blank Century. They add to that adventure feel to the story.
(Can't think up of much more so i'll cut it short)
8. Its drawn in a style where it perfectly fits the characters and their personalities.
9. Goes at a quick pace and has super massive awesome arcs.
10. After re-reading one piece a couple times you start to notice Nami growing "bigger" if you know what i mean