My take: They're all right.
Malkin is absolutely correct that the GOP must prove it is born again on fiscal responsibility. If the Republicans don't prove it, then the Tea Party will swoop in like the Shadow Host of Dunharrow in "The Lord of the Rings" and mow down the Republicans like so many dimwitted orcs.
Krauthammer, I think, is uncharacteristically shortsighted. Politicians not only need mandates, they need to understand what their mandates are. Otherwise they tend to think they were elected for their sheer personal awesomeness. President Obama, somewhat understandably, thought he had a messianic mandate to push a hard partisan agenda from the left. In reality, voters thought his mandate was to be "not Bush" and to then govern from the center. He fulfilled the first part and ignored the second entirely.
It's true that running on something rather than nothing might cost the GOP some campaign victories, but running on nothing would deny them even more policy victories. Sending Republicans back into power without a clear mission is like sending teenagers to Vegas for a school trip without a chaperone. Sure, they'll check out the museums.
As for the argument that the Pledge doesn't go far enough, that's obviously true. But it's also true that the Pledge is far, far more ambitious than the Contract With America was.
Moreover, the fact that it garners support from across the GOP caucus is a good sign, not a bad one, not least because it shows that the GOP can reach out to both the tea parties and to independents. Obama and Pelosi's alienation of independents is destroying the Democratic Party right now. Why should the GOP emulate that strategy?
Conservatives shouldn't look at the Pledge as the sum total of the Republican agenda. They should see it as the opening bid.