It's not that I don't think Colbert is as funny as Letterman. He's at least as funny as Letterman.
Nor do I think that it signals a dramatic and undesirable political shift on CBS late night TV. Colbert spends more time on politics than Letterman, but I bet if they were both congresscritters they'd vote the same way 90%+ of the time.
I just happen to like The Colbert Report, and to think that a more generalized, network-friendly late night show featuring Colbert will not be as good at the things he's good at.
One of my favorite Colbert segments: