It could have been dirtier

As details emerge of the inner workings of the campaign, it is clear that the negative advertising could easily have been much worse, had the candidates not set up clear guidelines.

Per Newsweek, McCain refused to run any ads attacking Jeremiah Wright or Michelle Obama. He thought that bringing up Wright would smack of racism (it might have) and going after Michelle Obama would have been quite awful (families are generally considered off limits). He also made it a point not to attack Obama's lack of military service.

Obama in a way reciprocated by not directly attacking McCain's age nor his wife.  He did not talk about McCain's previous divorce or the Keating Five scandal from the 1990s.  

Like I said, it could have been worse.