I was trying to think of a funny ad, that exemplified my point about tone. While making lunch I remembered this ad from Ned Lamont, who ran against Lieberman in the 2006 Democratic primary. [youtube=http://www.youtube.com/watch?v=dWSN2zbydKw]
Ned Lamont has a messy desk... What makes this ad work is that it takes itself seriously, overly so, but seriously nonetheless. It sounds and looks like a classic attack ad, but the contrast of the absurdity of the claims with the seriousness of tone make it funny.
Take a look at this scene from a classic "I Love Lucy."
Lucille Ball doesn't play the humor of the moment, she plays the reality of it. What do you do when the conveyor belt goes too fast? It's funny because it's played straight, when you play for laughs, you often don't get them.