The variety of genres in European comics, and particularly Franco-Belgian comics, is enormous - about the same as literature or cinema. I think that is the major difference. Growing up with all that diversity, few European kids feel attracted by super-hero comics. In my opinion super-hero comics are among the most boring and repetitive there is. I didn't mind them until about 8 years old, then got seriously fed up. I don't understand how so many adults still read them or watch cinema adaptations (except Batman, which is more realistic and closer to the James Bond genre).
I have always been a fan of realistic, historical comics. But those that I liked when I was a child aren't the same as those as read as a teenager or as an adult. American comics don't have a clear-cut separation between children and adult publications.