Thanks for calling some-thing lame that a lot of people on this forum obviously likes, even loves.
Admittedly, I admit it! It takes a while for One piece to transform from just good, to really great. It takes a loooong while.
Honestly, if I am to be really honest, I think One Piece first manages to find its true emotional core in the Drum island arch, and it first manages to show just how amazingly great it can be in the Water Seven arch.. and that's a good while into the show/Manga.
There are more than one reason to like it though, if I went really into it, I would write an entire essay that you people would just pass, so I must try to be as short as possible.
So well personally, I think it is funny as hell, has great multi layered characters you get emotionally attached to, amazingly imaginative and wild story lines, a great sense of childish adventure, genuinely emotionally deep felt story lines, and most of all. It entertains me, it entertains me a lot.