Senator McCain's sense of humor has been both an asset and a liability over the years. It is certainly part of his basic appeal, and it is one of the reasons he relates so well at town hall meetings as his humor creates an immediate connection with the audience. His detractors will note that some of his jokes have been inapprporiate, and in some cases that's true, hence his humor as liability. It can also cause problems because in print, jokes often don't come across the way they do in person. For instance, during the primaries Senator McCain jokingly called a student who asked him a somewhat inappropriate question 'a little jerk' (clip below). There was quite a buzz in the papers until people actually saw the clip, and it was quite evident that he was joking.
So now comes the humorous and sarcastic McCain ad The One. In this case humor turns out to be the perfect remedy for an over-inflated ego. It's hard not see lines like, "We are the ones we've been waiting for" or "a light will shine down from somewhere" as flat out funny. To add to this Obama's reaction is priceless. His stern condemnation of the ads 'juvenile' just shows his inability to laugh at himself. You know that if you make a joke at Senator McCain's expense he'll likely be the first one to laugh (as he has done numerous times when teased about his age). On the other hand, Senator Obama blew it by taking this joke and himself so seriously.
WMUR - Little Jerk
Humor as a Remedy for Hubris