Sometimes I’ll see people say that western comics beat out manga because when they actually are written to be sophisticated they do so in a much more mature and literary fashion. Granted, Miyazaki’s Nausicaa is richly dense in this respect but he’s the exception that sort of proves the rule as he’s greatly influenced by European comics.
However, I think that the greater strength of manga in general is that it manages to marry strong ideas and deeper philosophy with a very clear, conventional story-telling style often meant for young readers. While Naruto is indeed a children’s comic, no one should be ashamed of reading it while they’re above the age of 10 as it carries (and sometimes loses) interesting themes of redemption and friendship. You don’t have to dig deep to find out that Naruto is trying to fight 12 years of neglect and depression throughout his own series, or to know that Oscar from Rose of Versailles has to struggle with the conflict that arises from her trying to understand her own gender. This is not a bad thing.
I already have an exception, as I think this may be why Avatar: The Last Airbender is so appealing to its fans (which includes myself) as well. While it still feels very western, it is similar to manga in the sense that there are many themes running throughout the show but they are not obscured and require multiple viewings to get most of them.
Sure, they’re not Grant Morrison or Alan Moore, but they don’t need to be.