Since you've asked, I'll present my own personal opinion. I've always believed that manga is quite a bit different than American comics. Manga are commonly presented in either weekly or bi-weekly instalments or in collected books. Therefore it's not as necessary for any single issue or episode of a manga story to have the distinct set-up, rising action and climax so common in American super-hero books. Manga often tends focus more on setting and establishment than do typical American comics. It's much more common in manga than in American comics to see panels depicting simply abstract images or establishing views of buildings or walls or other surroundings. American comics, largely, have a tendancy to try to maintain a sharp focus on the characters by keeping the characters centered in the panels as much as possible. American comics, particularly super-hero stories, usually tend to focus on depicting action in wide-shots, so readers can follow the movement of the characters. Manga tends to often aim for a more general atmosphere of kineticism, often illustrating action in extreme close-ups that obscure the exact motions of the characters. It can be said that manga also encompasses a much wider scope of subject than do American comics. I don't want to offend fans of American underground and independent and alternative comics. I'm certainly aware that the American comics industry offers much more than big guns, big muscles, and big breasts, yet I don't think the American comics industry can claim the range of diversity of manga when there exist such things as economics text-books in manga form, such as Japan, Inc. I've also heard it said that American comics simply don't match the dynamic panel lay-out of manga, and that manga strives to present itself as a complete, readable story even without the dialogue. Whether either of these statements is true, I'll leave to the reader to decide.
