Bennie Grezlik, David B. Carren, Lou Antonelli, Lawrence Person and Stina Leicht in the "Through a Lens Darkly: Why are so many current Spec Fic movies so darn dark and depressing?" panel. Stina has said more on this panel in her blog (http://stinabat.livejournal.com/236628.html), but I'll add a couple more points that caught my attention.
Lou Antonelli says that the grim future, promised so many times, hasn't come true so far; for example, Japan, a country that was nuked twice in world War 2, came back stronger than ever, and enjoys a higher standard of living than the West. So Antonelli wonders if dystopia trend merely reflects Western concern about the decline of Western civilization. (Stina says "yes".)
David B. Carr thinks post-apocalyptic novels or movies appeal to people mainly because everybody secretly identifies with the survivors. In most of those scenarios only a few selected people survive, and fans of such movies may be unconsciously thinking that (a) that most of the humanity should die, and (b) they would be among the chosen few who would survive. That's not have a very generous opinion of the fans of this genre, but there may be a grain of truth in it.
Here are some of my blog posts from ApolloCon 2010: