@jadiesama:
in my opinion, it's all sequential art. It shouldn't matter where it came from as long as the story is good and the art is legible. Unfortunately i'm pretty picky about what i read which is why i've pretty much barred any and all shojo stuff since i'm not a fan of neither romance, nor the oft non-existant backgrounds, nor the multitude of effeminate men, and alot of Superhero comics, but moreso for the fact that if i start a series, i want to read it in its entirety, which is pretty hard nowadays. The exception to the latter will be the Dark Knight Returns if i ever get around to finding it.
It's like porno. You can't define it, but you know what it is. XD
The differences seem to be largely a "format of publication" thing.
-The "two-tier" marketing. American comics are sold usually in monthly "buy it now or you're stuck." While you can buy manga in the original weekly/monthly package, the tankoban followup allows you to affordably catch up and keep up with a series even if you miss an issue.
-The series tend to end, usually in under 10, and almost always in under 20 years. Many popular American series date from before the Viet Nam war, if not WWII.
-The manga I've seen (which means ASJ plus one or two series) is black-and-white dominant, and the western series tend to be mostly colour pages.
What I find draws me to manga is the first point. I like that I could easily go on the web and get every (English) OP volume, read and enjoy the whole story, and I'll bet the whole series, all 70, 80, 90, whatever books will cost less together than one copy of the first chapter of Batman or Superman. :D