-The story is good.
-The power-ups are believable
-The alternative art style is very awesome.
-The protagonist is endearing, and the antagonists are all very interesting.
-Oda gets into some fun elementary philosophy, such as "who determine justice," what price people pay to be free, etc...
-Unlike Bleach and Naruto, One Piece's chapters do a good job of cramming events and action in; it's a satisfying read that doesn't make you say "is that it?"
And, to top it all off, both the anime and manga are dealing with a war right now.