American comics are for some part good. Many tell unique stories but you get the faint hint of the author holding back because he doesn't have faith in his audience that they'll believe everything unless there's some dose of 'reality'/'realism'. Most aren't that good. Most are a pain in the ass to get into.
A good example of how the American comic market is like is writer Mark Millar. Sometimes he writes some REALLY good stuff (ex: Kick-Ass, Ultimate X-Men, and Red Son from what Ive heard) and then he writes some down writer awful stuff (WANTED, Ultimate FF4, etc, a plethora of bad, over-the-top shitty writing). And that's how the American market is. You'll have your Dark Knight Returns, Watchmen, Batman Year One, and such but compared to the over larger catalog, there's very little cream of the crop to enjoy. And alot of it goes on wayyy too long. Ex; Ultimate Spider-Man and Ultimate X-Men should've ended a while ago, but now they're repeating the past and forcing it continue into further issues with too many cameo characters and deteriorating story.
Manga however has a culturally accepted method of full-fledged creative freedom. Thus is more richness to the storytelling in manga and its not as overwhelming as some over-detailed American artwork in typical comics. Its stylisticness and looseness allows it to be digested and absorbed better. It gives the subtle effect of drawing you into a different world without taking away your belief of it. I will say that alot of manga isn't THAT good and some of them have such popularity that it baffles me (i.e. One Piece, D-Gray Man, Reborn!, Prince of Tennis) but I guess some people like cliche, shallow entertainment, same as American comics (however the market's been going down slightly lately which is why they created Ultimate comics lines of characters like SpiderMan and XMen and Ive heard manga sales in Japan are slipping too and probably for the same reason).
Basically the difference is that American comics are rigid and dark and pretentious and sometimes hard to get into (like Dark Tower) because of their somewhat niche stories but manga has somehow more universal appeal with its stories. And most of the comics that are really good in America have that loose, stylistic aesthetic of manga (like Ultimate SpiderMan/XMen, Dark Knight Returns and other Frank Miller work, The Long Halloween).
And alot of American writers lack imagination because of cultural expectational norms and self-indulgence which is why they write about the same characters again and again even though by this point there's not much elsewhere to go with them. One reason that I appreciate manga -- it ends.
Hope this helps. Special note is that Im a film buff/major in uni so Im a harsh (yet fair) critique of story so Im sorry if my harsh dislike of certain comics puts them off.