One of the other factors is your niche, and the amount of ads targeting your niche. For instance, I think icanhascheezburger gets very high ad click throughs an

One of the other factors is your niche, and the amount of ads targeting your niche. For instance, I think icanhascheezburger gets very high ad click throughs and impression rates, because of how targeted they are. I can't find the article, but I think most of their ads are for pet products, since people looking at cute cats (and dogs, for their sister site) are predisposed to owning, or looking to own a pet.