Anger
I've heard bits and pieces about rumors surrounding Stewart's replacement decision, and I knew that there was some bitterness over his selection, but, now that I see Samantha on her own show, I think it's pretty obvious that she's being driven, to an extent, by anger. Her on air persona has always been pretty angry, but, she's kicking it up a notch here.
Which, I think, works well for the comedy. Hell hath no fury like a female news correspondent overlooked for a promotion :)