Timothy Noah argues against the Stewart-Colbert rallies:
Stewart-Colbertism scorns extremism of all types, but especially conservative extremism, and most especially conservative extremism driven by ignorance or religious fundamentalism. It is mildly critical of liberalism, but mainly for failing to combat conservative bombast more effectively. It endorses, implicitly, whatever liberal consensus has managed to survive these past 30 years, but isn't terribly interested in the details.
All this works well as humor, but as a sentiment shouted through a bullhorn to thousands stretched between the U.S. Capitol and the Washington Monument, it will translate into, well, judging other people for what they don't know. It will do so no matter how much everyone laughs.