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Link evaluation. We often use characteristics of links to help us figure out the topic of a linked page. We have changed the way in which we evaluate links; in particular, we are turning off a method of link analysis that we used for several years. We often rearchitect or turn off parts of our scoring in order to keep our system maintainable, clean and understandable.
Many or most of Google's changes over are made to catch and devalue people/sites that play the system with bad information to get on top.
I was also thinking that this link change may have to do with allowing the surrounding text and even the title of the linking page affect their understanding of the topic for the target page. This seemed like a potential source of noise and error, as far as I'm concerned. With all the evolution in their on-page processing, they might well have decided that this part of the algo just wasn't helpful anymore - they already had the topic pretty well nailed from more direct signals.
[edited by: Robert_Charlton at 10:10 pm (utc) on Feb 29, 2012]
[edit reason] removed forum link [/edit]
My contribution - this explains why I saw links to affiliate sites result in seriously lowered rankings for those pages on or about Feb 6th. Time to re-evaluate how you link to affiliate sites.