As I said, you don't have to watch it completely, just let it build up a bit and see if you like it. It has over 600 episodes so it can quite overwhelming.
Oh you stream the videos? I usually download everything so didn't even consider that streaming has area limitations. Anyway, I wouldn't call One Piece childish, at least not more than Bleach. Though, I guess it kinda depends on the person. To be fair, I did need to watch like 100 episodes to get into it but after that I got too into it and started reading the manga.
Since you haven't watched all that much, a popular long series is One Piece. A few others, Sword Art Online, One Outs and Fullmetal Alchemist: Brotherhood. Furthermore, my animelist might give you some others if you sort by score.