Then there's the creativity of the series' plotline and setting. People make the mistake of assuming it's a generic anime in this respect but any fan could tell you that the show covers a marvelous range of different locations (islands built in the sky, prehistoric islands, desert islands, and more) and that it has several very unexpected plot twists and plot conventions the likes of which are hard to find in other shonen anime. The world of the story (including its unique geography and its zany, but thrillingly powerful inhabitants) is remarkably vast and a treat for the imagination. It's not hard at all for a fan to be utterly swept up in it! There are also several interweaving subplots behind the main goal of the show (which is to find the One Piece treasure) that keep the show alive and fresh (as opposed to, say, Naruto's perpetual quest to save Sasuke). Not to mention, at times, it can be quite mature and meaningful in a poignant, though non-melodramatic way. And the comedy can be just plain hilarious!
As for the show's artwork, this is more controversial since some people simply detest it since they usually find it to be too "childish". However for fans of the show the artwork is viewed as being unique and, personally, I feel it captures the adventuring, free-spirited air of the story brilliantly.
Now, to more concisely answer your question, I would have to say what ranks One Piece above other long-running series is its quality. It avoids becoming repetitive, constant attention is given to character development, progress in the plot's development (while slow) is continued constantly, the thematic content is varied and meaningful (it deals with all sorts of issues like greed, corruption, even slavery), its fillers are kept to a minimum and are in fact also rather enjoyable, and it maintains its own unique style.
Just as a warning though, some people find that the first arcs of the show are dull (meaning almost the first hundred eps) and, so, conclude that the entire series must be dull. But really it keeps getting better and better (unlike most long-running series that seem to just sort of tail off in their later episodes). Right now, after 400 episodes, we're reaching what almost all fans promise is going to be the most epic arc to date! So you see, it's really the sort of show that improves with age and can captivate any fan. It just might require some patience in the beginning (though I personally loved it right from the start).
