I think once you accept that this kind of fissioning into liberals and progressives is a periodic feature of American political life rather than a new and sudden betrayal, you have to look at a magazine caught in the middle in a different light. Obviously some of its long time contributors will be caught in transition. To take the magazine's worth is to see which way it drifts -- whether, once the choice becomes unavoidable, it chooses to become born-again liberal (feeling revitalized through one's lust for the enemy -- a stance now called neoconservative) or progressive (feeling that hatred of the other is not only inherently unprogressive but counterproductive). And the only way to do that is to make a distinction between what the magazine publishes and what it doesn't. Which means, in other words, to make a distinction between what their contributors publish in the magazine and what they publish elsewhere.
If a magazine like the Nation publishes these people when they write progressive articles, and pointedly doesn't publish them when they publish neoconservative articles, then it's a progressive magazine. I don't see what else could possibly be asked of them under the circumstances unless you want to institute democratic centralism. And it's not an accident that a popping fresh neoconservative like Hitchens felt he had to make a dramatic break with them. He was right. He and they have fallen on opposite sides of a new divide.
Of course none of this bears on the boringness of the Nation, which is a different problem. But I have to say, IMHO, this fissioning thing has actually made it appreciably less boring. Looking back over its history, that might always be the case. It might be that the Nation is always worst when liberals and progressives are united and in power, and best when they are at each other's throats.