I haven't followed up on the comments, but I do find the whole debate interesting from both my perspective as a reader, and writer of both YA fantasy and now (debatably) more adult fantasy. I've been told anecdotally by more than one author that the YA market is much more likely to bring in better returns.
I ate up SF/F books as a teen. I think it was the sense of wonder, newness and adventure that I loved about the stories I read. For some reason, I have a harder time finding that same sense of adventure, maybe even playfulness in the much of the adult SF/F fiction I read. Maybe I'm not reading the right books. I got really depressed last year, because half the books I read (SF/F or otherwise) just didn't set off that spark within me. Maybe I'm just getting jaded. But I don't think so. I've gone back to read some of those books I loved when I was younger and they've still got "it". Whatever it is. YA fiction often explores themes as dark and dramatic as adult fiction. And yet, I find it much more hopeful overall than many of the adult works I've read recently.