Man…I'm seeing myself addicted to One Piece, as I've never seen to any other manga ever...I mean, I'm not that young...and I've been into anime and manga since I was a kid (nearly 20 years, now... XD).
One of the main things that contribute to this, I believe, it's the characters' charismas. Odacchi does know how to make characters whom we love or hate. As I "sell" OP to my friends, one of the strongest points in the series is its creator ability to control our emotions: we get pumped up when he "orders", we get worried when he orders, we laugh our asses off when he orders, we cry rivers when he orders, and so on.
The main cast is another thing: it's not a small bunch (if you consider other mangas like Saint Seiya, which has four mains and an almost main), we have, up to the moment, 9 characters in the main crew (not to mention the ship itself), and I can point out at least one characteristic of each of them in me: Luffy's single-mindedness (sometimes), Zoro's short fuse, Usopp's...er...extreme caution with himself in regards of self-preservation XD, Nami's lust for money, Sanji's gallantness toward any pretty female in the vicinity, Chopper's gullibility, Robin's capacity of naturally making creepy statements as if they were normal, Franky's lack of capacity to disguise his emotions, and Brook' fondness of company. Not to mention the thing in common in them all: the strong link to their nakama. And this is for me, I'm sure that everyone here might have this or that, be it from the main characters, or any other character in the series.
And about the story itself, and the beforementioned Oda's skill of controlling our emotions, I can pinpoint few things in pop culture that really made me cry (to a point that some of the chapters/episodes, I realized that I'd best finish reading/watching that alone at home, so I would not be embarrassed in public!) XD
I mean, it's not hard to move me (my Franky side XD), but to cry like I did at Nami's and Chopper's stories, or a certain event which happened at the end of Enies Lobby, just things like Rurouni Kenshin's first OVA series, or Hotaru no Haka (Grave of Fireflies). ^^'
What the series did impact in my life, actually, was that it gave me strength to leave my homeland, my friends and parents, in pursuit of my dreams. Strangely, I did think a lot about OP when the decision time came. And, even if foolishly, I'm really grateful to Oda-sensei nowadays because I do believe that the "final kick" to embrace the quest for my dreams came from him. :)