"Of late, the trend in superhero comic book art has been a move toward a more painterly style, as expressed best (perhaps) by Alex Ross, rather than the simpler line drawings of our youth. Why is that?"It is a reflection of a change in status of the once maligned art form. Comic books were printed on the cheapest of paper with line drawings in order to entice young boys to spend their allowances. But then three things happened.
With the sentimentalism of the baby boomers, comic books and baseball cards went from toys to serious collectibles. The price of older rare comic books went sky high and brought up the monetary values of not so old, not so rare editions. With this increase in worth came an increased sense of value and the notion that the comic books would have to live up to this new found status.
At the same time, popular culture was being elevated from its status as low art. No doubt the works of Roy Lichtenstein made everyone look at comic book illustrations differently, but in general we started looking at everything from television sitcoms to comic books with a more critical and sociological eye, trying to make sense of what our art forms said about us. My Grateful Dead book is part of this tide that takes what was once held to be beneath intellectual treatment and allows it the pride of place of other more standard topics of deep conversation. The works of artists like R. Crumb, Steve Gerber, and Gilbert Shelton put comics in the political arena while not lapsing into the separate world of political cartooning. So, it became a more serious place.
Indeed, we now have a new category of "graphic novels." A graphic novel used to be a story with elements for adults only, now it is a comic book for grown-ups. Works like Art Spiegelman's Maus took the medium to a completely different sphere. Now, you see complexity and sophistication not only in the artwork, but in the plots of series like The Watchmen that were not there a couple generations ago.
Finally, there is technology. Art is now done not with pen and ink or paint and brush, but with a mouse and software. Better than simple line drawings can be easily generated and disseminated free through on-line comics. If you want to charge for your art you better be able to do better than what is free.
Any art form will have movements and I think the comic book has emerged as a legitimate art form. I think what Gwydion points out is simply one trend among many that are and are to come.