To be fair, George Bush was fairly self-deprecating, and kinda funny. His presentation was also on YouTube as a single ten-minute presentation.
I've thought a bit more about the two presentations. Bush clearly got more laughs, and was willing to make fun of his foibles and those of his administration ('I survived the White House shakeup!') while Colbert really did meet much more dead response. I think Colbert had a difficult job: balancing the character he portrayed on the show with presenting to the attendees, a very specific audience. His routine was wrong for the audience to which he presented. I sure didn't laugh very much either, as I was sort of stunned with his jokes, which *were* funny, but impressively scathing. I applaud his choices, and hope he doesn't find himself on the unemployment line like Bill Maher and Phil Donahue. Maybe they'll just demote him back to working with Jon.