The problem with one piece is only that it starts slow. It takes awhile before you get a feel for the larger plot and get attached to the characters. Not everyone has the time or patience to spend 100 episodes getting into it like me. If you got hooked after episode 1 then that is fine, but for a lot of people it took until alabasta and you shouldn't hate them for not wanting to watch 100 episodes before getting into it. To comapre with bleach, most people say watch the first 20 and then you will like it and for naruto its even smaller like 8-15 before zabuza shows people what the show is about.
The common theme seems to be that the longer it takes until the show starts giving meaningful villains, the longer it takes someone to get into it. Some shows/mangas want to develop more quickly then others, don't blame people who don't want to spend forever until they start feeling entertained, I mean the whole point is to spend time being entertained right?