Yes, speak truth to power, we all understand how badly they've failed at that. But I think for a lot of them the problems is that they really don't understand the issues well enough to know when they're being lied to.
That set of clips Stewart had, and the cue cards, showed an incredible amount of preparation. With most politicians it would be easy to look at their last two weeks' worth of public statements and get a good idea what their current talking points are. So pull their previous speeches. Find the times they said the opposite. Call them on it live, while they're sitting there looking at you.
I've seen him interview authors, retired generals, foreign ambassadors and heads of state. There wasn't once that I didn't thing, "Damn, he really knows his shit." Whether it's amazing interview prep, or he genuinely keeps up-to-date on all these issues, he is always prepared.
That's what journalism is supposed to be. Not just looking good on camera, speaking clearly, and reading the teleprompter.