Stephen Colbert is no David Letterman. Sure, Colbert can do irony; he can mercilessly mock those deserving of mockery, and satirize the powers that be; and he can aim a sharp barb at a politician or a movie star or a network president; and he is more than happy to make a fool of himself. But, at least during Tuesday night?s much-hyped debut of the Letterman show?s successor on CBS, The Late Show With Stephen Colbert, there was nary a sour whiff of the world-weary psychic struggle, the undercurrent of self-flagellating insecurity, or the deep suspicion of human motives that fueled Letterman?s comic genius and also stoked, especially in the latter years of his more than three-decade career as a talk show host (including 22 years on CBS), his legendary discontent.