The only weakness of One Piece is that it doesn't have an exciting beginning unlike Naruto. However unlike Naruto, the series improves drastically as the story proceeds, while Naruto is slowly but surely degrading. Due to the unexciting beginning, many people are unwilling to keep on reading/watching One Piece and thereby resulting in a smaller fan base compare to Naruto or Bleach. Yet if all those Naruto tards out there actually sit down and spend couple hours on One Piece, they will love it.
Perhaps that's also why One Piece is starting to get a bit more popular now days in North America. After all, I am in no position whatsoever as to lecturing about how slow people are. It took me 2 years to realize that One Piece is the greatest series in existence >.<.(Started two years ago, dropped it, started again almost two years later, loved it.)