Once upon a time, I started watching both Naruto and One Piece; since they were both well regarded shonen fighting shows, I expected to enjoy them more or less equally. Instead and rather to my surprise, I soon lost interest in One Piece but kept watching Naruto for much longer. Afterwards, I spent some time thinking about why and what the difference between the two shows was, because it was not as simple as disliking one set of characters while liking the other.

I eventually decided that the crucial difference was that right from the start, Naruto's story was always going somewhere, always moving forward. Naruto's desire to be Hokage drove him to action right from the first episode, and everything kept happening and developing from there. By contrast, One Piece's story seemed to be sailing around as leisurely and aimlessly as Luffy himself, and without the story going anywhere, the rest of the show wasn't enough to keep me watching.

(The breaking point for One Piece came when I realized that I actively didn't care about one character's heart touching background that was about to be explored. I just wanted something to happen.)

For lack of a better term, I've taken to calling this 'narrative momentum'; the degree to which things are actively happening, being reacted to, and pushing the overall story forward. Note that not having narrative momentum is not a bad thing in and of itself; it depends on the genre and the story that is being told. There are lots of stories (and entire genres) where narrative momentum would actually be bad, where things benefit from the story moving slowly and quietly.

(As Naruto demonstrates, you don't have to focus only on the main story to keep narrative momentum; you just do flashbacks and character backgrounds mostly as a consequence of them affecting the main story, so that they sustain the momentum instead of taking away from it.)

For me, lacking narrative momentum is different than moving slowly, as unfortunately demonstrated by Naruto (among others; it's a common shonen fighting show problem). One reason I stopped watching Naruto was that while it had narrative momentum, big events like fights just dragged on for too many episodes.