Nice discussion, but may be anyone has some data backed observations regarding the nature of this update? Shared IP issue is certainly a bug and not what this update is about. All adsense theories are very unlikely as well.
I tried to compare a few previously well ranked websites that went down vs the ones that retained their positions. It seems to me that there is some correlation between the (lack of) diversity in backlinks anchor texts and loss of rankings. It has been some time already that Google is favoring a more uniform distribution of anchor text, but now it looks like they make a really big deal out of it.
One observation that I find interesting: I have a site that at some point in the past received a huge number of unnatural links with an anchor "blue widget". Its backlink profile became skewed with a highly unproportional number of backlinks having "blue widgets" as an anchor text. It was penalized for that particular keyword and haven't ranked well for it for a year already. However, it used to rank very well for "round blue widgets", and "<other> blue widgets". What happened in this update is that it was pushed down by 1-2 pages for all these tri-gram keywords it used to rank for.
Considering all this recent talk about the overoptimization penalty, I am wondering if that's one of the new things introduced by this update.
Does anyone see smth similar? Share.