I've just been reading a cool article on SearchEngineLand about advertising and the different ways ads are killing SEO.
They refer to "top heavy" which apparently was something that came out in 2012 but yeah I am slow to learn some things... From the way I understand it Google Adsense was allowing a maximum of 3 ads showing per page, however now they have just changed how this works.
Now you need to have a lot more content than ads which makes complete sense to me. I go to a website to read content, not to get bombarded by advertisements!
So make sure you have loads of quality content and then some well placed ads in moderation....