What really draws me to it is arcs themselves. Each one is nothing like the last, and brings something new to the table.
The characters…while most of them have the basic personalities of a lot of other shonen, they're developed in such a way that you don't notice it and they seem more of themselves, and not based off of other characters. Then I like how oda sets up the characters each with a dream that they want to accomplish. This lets you think more of whichever character you want as the main character. Sure Oda sets up Luffy as the captain and the main character at first, but from there on out, he never does much more with that then letting Luffy fight the strongest guy every arc. So you're more free to think of the characters as you like in the story. Unlike say Naruto, where Naruto is basically defined as the main character.
The villians are some of the most amazing I've seen.
It's hilarious
And I could go on and on.