sanjuu - 2:42 pm on Apr 12, 2011 (gmt 0)
Shaddows - I agree that it's a combination of factors, like you said it's a complex algorithm that has taken a while to develop.
It seems to me that the overall goal is to reduce the number of sites ranking towards the top of the SERPS that appear to be using 'thin content' to rank well in order to benefit directly from display advertising/adsense.
You've probably already discussed this to death in the US discussion (and I'll read through it all), but I do look at the role of this change as a 'negative-impact event' - effectively penalising sites that Google doesn't want to rank well (with their think content to drive traffic to their ads), and by default the sites that aren't seen to be doing this will fill the vacuum of spaces that are left. Ranking well is about doing everything well, it's about doing it better than everyone else you're competing with.
It would be simple for Google to use the ad blocks as a 'red flag' - and then follow up this flag by looking closely at the site content, etc., and then determining if they think the site is a low-quality/thin-content/crap site.
Of course, this doesn't mean every site that has been negatively effected has lots of ads, or that all sites with lots of ads have been hit.