What I like about One Piece? Hell, everything! It would take me a whole day to write it all. So I won't..
.. or not.
THE STORY/PLOT - Even though you have absolutely no idea when's the show/manga going to end, you just know that there is something that the Straw Hat pirates must do, first. You know that they must finish this task, otherwise Oda simply can't end their adventures. He understands that, and since he decided not to stop the series when they were supposed to (originally One Piece was supposed to last for 5 years, if I'm not mistaken) now he must find a way to BOTH entertain us AND try to make One Piece as short as possible.
Other than that, the story is awesome! The arcs throughout the story can always keep you entertained.
THE CHARACTERS/THEIR DEVELOPMENT - One of the most noticable things in One Piece. The characters and their development throughout the series. Take Luffy for example. At the start he was simply relying on his luck and his Devil Fruit ability. But NOW he has mastered Soru and is using it flawlessly. I am sure that Luffy's fighting skills will develop even more. Now let's take another character.. like Zoro, for example? At first, he was also a bit like Luffy, but he actually relied on his Santoryu and the skills he picked up when he was learning to fight with Kuina. Now Zoro (after joining the Straw Hat pirates) is trying his absolute best to BE the best! His goal is to "never lose" and one day defeat Mihawk (I'm sure he'll do it, btw) and truly become the world's greatest swordsman.
And that's exactly what I like about the One Piece characters. Every one of them is unique and has his own style in doing things.
THE HUMOUR - Oh, you just can't deny that when it comes down to humour, One Piece tops most of the mangas/animes. Whether it's gonna be a "^_____^" facial expression, or simply another "oopsie" by Luffy.. you'll always find a reason to laugh. That's one of the main reasons why I love One Piece. Gold medal for One Piece, when it comes down to humour.
THE FIGHTS - My personal favourite part of One Piece. The fights. I mean, how is it possible (in real life or not) for one man to be so dedicated to their friends? To a point that you're actually willing to die, but save them. Just look.. none of the Straw Hats are actually fighting for themselves. There's always a strange reason why they're doing all of this. You see, they fight for justice. It's pretty much always this way: "Hey, you hurt my friend.. time to kick your ass!". Now THAT's epic!
These are the main reasons why I love One Piece.
I just got lazy, and stopped writing. 
cough damn, long post cough shutting up cough