Tom Goldman said:
The word "comic" itself could be part of the reason comic books are criticized, giving the medium a child-like connotation. Comic books are anything but, especially these days with the average comic book marketed more towards adults than children. Manga could be an exception, though many series have mature content not meant for kids.
As someone who has actually studied Manga I can tell you that Manga specifically is NOT an exception. In fact, from it's sheer origin Manga tends to be a lot more about philosophical, psychological and even metaphysical concepts. You have authors that study these things extensively just for the sake of writing their stories, while western comics have a history of relying much more on the boyish "white knight"/power fantasy.
That said, it's always a gross over-generalization to say "manga>comic" or "comic>manga". It depends a lot on authors. Alan Moore [http://en.wikipedia.org/wiki/Alan_Moore] is a god amongst writers everywhere, but so is Yoshiyuki Sadamoto [http://en.wikipedia.org/wiki/Yoshiyuki_Sadamoto] creator of Neon Genesis Evangelion. At the same time you can find copious amounts of crap on the west and east alike.
Long story short, don't discount Manga, you'd be surprised.