One of the most notable aspects of manga is its wider range of genre and target audience/demographics.

There are far more selections for females--elementary school, junior high, high school, college, and adult--in manga than in English-language comics. Shoujo manga refers to a relatively small subset of this group, but when used outside of Japan frequently indicates the entire range. And no, they're not all romance stories. They include thoughtful science fiction, sports action, fantasy, etc. etc. (Oh, and also soft porn. So much for the Euro-Americocentric idea that women are turned on verbally, men are turned on visually.)

Manga artists (usually, but not always, also the writers) tend to employ more advanced storytelling techniques. For an interesting analysis, see Understanding Comics by Scott McCloud. Other interesting books include Manga! Manga! and Dreamland Japan, both by Frederick L. Schodt.