i could speak on behalf of my wife.
at first she never liked one piece, back when she was 16. she loved fairy tail....because of cool characters and friendship and comedy.
when i made her watch one piece, she did not like it, until we got into the arlong park.
she said she now saw the reason why fairy tail is being shitted upon and why one piece is popular.
complex story, great world building, exploration, all characters get character development, villains really got a reason why they do things, the world of one piece is really big, the fights are needed in the story etc etc.
she said she now sees that fairy tail is a really good series for you if you like childish stuffs but once you grew out of it, you would see that there are waaaaaay more other mangas and anime that are worth watching.
and a good shounen series is not about comedy and coolness