I can't presume to speak for every OPfag ever, but here's why I, an otherwise highly jaded and cynical weeaboo, enjoys something like Win Piece:
I like One Piece because it may be hyperactive shonen, but by shonen standards, it really "keeps it clean" so to speak, in terms of striking a fairly healthy balance between FIGHT FIGHT FIGHT and GAR GAR GAR POWER OF YOUTH AND DREAMS. The fact that it has a genuinely neat, bombastic art and aesthetic style is just the icing on the cake. (I also always kind of liked how relatively empowered Nami is by shonen standards -- even if they mostly set her up as a gimmick-fight, they actually let her be smart and capable in other aspects rather than being a glorified cheerleader for the snarling male leads.)
I also like it because I'm something of a sucker for the OG hyper-GAR shonen shenanigans rather than the snarling teenagers of modern shonen. Luffy and friends play the MANLY TEARS card tastefully, well aware of the cheesiness factor all the while -- and it's always refreshing to see a true berserker hero like Luffy, who's just stupid and fearless and totally devoted to his friends without any token existential angst attached. No matter how jaded you may be, it's pretty much impossible not to love Luffy's single-minded zest for friendship and adventure.
Besides, you gotta love how it just gets sillier and sillier to describe to people who don't follow Win Piece. I mean, hell, just take Enies Lobby -- Luffy and friends board a big-ass sea train with a bunch of hoodlums and a cola-powered cyborg shipwright to rescue Robin from an evil bastard with a sword that turns into an elephant, jumping over a huge-ass wall with an army of pirates and sword-fighting a three-headed judge. Also, Zoro fights a sword-wielding assassin with a rectangular nose who turns into a giraffe. And that, my friends, is at least a little awesome.