I'm wandering through the comics-fanboy end of the blogsphere, which is mostly new territory to me. I came across this rant about the usual "why do mainstream comics suck so relentlessly" questions that were old and tired when I first went wandering through comics fandom ten or twelve years ago.
Aside from producing the above grand Old Leftist quote, which I am quite in love with, the writer also touches on a theory that I hadn't heard before. He suggests that the monthly pamphlet format of mainstream comics is geared to naturally produce eternal superhero serials. The monthly pace drives a simplicity and broadness of characterization and plotting, because the average reader is going to forget more subtle or elaborate constructs in the interval. The broadness lends itself to outsize dramatics, and thus, inevitably, to tights and mutant powers. Manga has certain similar issues; the installments are even shorter in the original formats.
I would argue that the weekly formats lend to a higher level of complexity, but for the fact that the weeklys are generally broader and more simple-minded than the higher-toned monthlys. Graphic novel releases would tend towards a higher tone under this theory; sadly, that's not the original format of most manga - the GNs are compilations from the phonebook weeklys or monthlys. Perhaps it has something to do with the universality, portability, and cheapness of Japanese GNs? They're much smaller than the bulky, floppy American graphic novel, much cheaper (average of under 700-800 yen as opposed to the American $15-$20), and much more predictable in their appearance.