Honest answer? For me it's the humor, and the sense of adventure. The fights are actually weak in my opinion. Oda's fights work best if they are humorous (Ussop vs Perona), or character development fights that tear a character apart, and force them to become stronger (Zoro vs Mr. 1.)
I like the research Oda has done on real pirates, (even though he really doesn't use it, you can tell he's at least studied up on the subject.) Some of the key pirates in the One Piece universe being named after historical pirates is a nice touch.
I like the insane long term planning he puts into his work. Characters shown on cover arcs, or in sillouete show up years later (real time years,) as major characters. I know some argue that Oda makes the story up as he goes along, but I honestly think he has this whole thing planned out in advance, he's just been distracted by minor arcs and keeps dragging things out. When Oda says he's had the ending planned out from the beginning, I believe him.
Oh there are better manga out there, but for a humorous adventure story, One Piece works very well.