In light of the recent 40th anniversary of Saturday Night Live, I wanted to make a quick distinction: doing an impression of someone isn't necessarily satire.
Dana Carvey was so funny as an impressionist that the entire show became mostly about repeating the tics of the person being impersonated. Any urge to satirize the person was diluted.
When Chevy Chase did his Gerald Ford impressions, such as the joke about Ford in a debate: "I was told there would be no math," he was also commenting on who Gerald Ford was. Carvey saying "wouldn't be prudent" every week for four years wasn't satirizing George Bush; it was simply doing an impression of him.
The Colbert Report was satire; Colbert was amalgamating and projecting the essence of Bill O'Reilly, Hannity, etc. If he just wanted to do an impression, he could have just shouted Pinhead! over and over again.
What Carvey did with Bush (and what Tina Fey did with Palin) was at best a caricature. Still takes talent to do it well, but it isn't satire. If all it took to be a satirist was impersonation, parrots would be the greatest satirists in the animal kingdom. Come to think, most pet shops are funnier than SNL...