The "Serious" mockery stems from the fact that his views are unaccompanied by any such work and are devoid of any critical thought. Klein, for instance, famously defended the President's NSA lawbreaking by admitting his Bush defense rested in blissful ignorance: "People like me who favor this program don't yet know enough about it yet. Those opposed to it know even less -- and certainly less than I do."
That is what a Serious Person does -- blindly trusts the President even when he breaks the law, and demonizes as Unserious those who object to presidential lawbreaking, exactly what Klein did when he scorned Unserious Nancy Pelosi in the pages of Time because she said that George Bush should not commit felonies when spying on Americans. Klein called objections to Bush's lawbreaking "civil-liberties fetishism" and said "these concerns [i.e., that Bush broke the law] pale before the importance of the program."
Klein also warned that if Democrats continued to object to illegal eavesdropping, "they will probably not regain the majority in Congress or the country," because "liberal Democrats are . . . far from the American mainstream" on this issue. The hallmark of Beltway Seriousness is the inability to do anything other than spout authority-worshipping conventional wisdom ("you better revere the President even when he breaks the law, and stop investigating him so much, or else you will lose elections") which is wrong time and again, while branding as "Unserious" anyone who challenges Beltway orthodoxy and, especially, who opposes too strenuously the High Beltway media and government priests. That is the essence of Beltway Seriousness.
Any pundit that could produce that kind of tripe, like Klein, really does need liberating. And any pundit that thinks he is serious because he can advocate either side or both sides of an argument proves only that he wants to show he is above others because he knows something the rest of us don't.