Yeah, its sluggish pace must be the #1 reason.
At the end of it all though, One Piece will be the most celebrated manga/anime ever. The story is such a grand one. I don't read it anymore, but I know a decent amount. All the islands and people Luffy and his crew have encountered, fought, and befriended makes for such an epic adventure. It has the depth of Naruto and the humor of DB, but much longer than both. Everyone who hates it now will jump on the bandwagon once huge events happen, like the final fight against Blackbeard and discovering the "One Piece". Those two events will get as much--if not more--media coverage--or than--as the revealing of Tobi's identity.
Make no mistake, Naruto will always be my #1 favorite, not only manga, but story EVER. I'm just being realistic. One Piece will triumph over all. Not in spite of, but because of its length.