hyperkik - 5:31 pm on Mar 6, 2011 (gmt 0)
I agree that many of these sites have too much ad space, but there are several notable exceptions that lead me to suspect that ad space is not the primary factor here.
The impact of the update is not necessarily site-wide. I have one site that was hit in part. The section that was hit (@40-50%) had very different html and css than the section that was not - but used the same advertisers and ad networks. Most of the pages in that section that was affected had fewer units per page than the section that was not affected. I've recoded the pages and should know, soon enough, if that makes a difference.