The last 2 case studies they released (these are meant as much to indicate what kind of sites they want as much as 'Hey look how much money you can make'). If you chat with the AdSense advisors, the advice they give now is MASSIVELY different than it was a few years ago. Heck, even the big push to try to meet up with more AdSense publishers. And if you listen really closely, the off handed remarks they make when you talk to them indicate a lot too. The reports people are making about being banned and why. People I know who have been banned and why.
I will bet money they are trying to nudge people a certain way. I have seen it before a few times with AdWords and certainly more than a few times with Search. If history in those areas holds true with AdSense (and I don't see why it would not), if nudging does not work, they will just chop out what they feel is not working.
One thing to remember with Google is their end customer is always the most important. They will and have sacrificed millions, possibly billions of dollars to keep those people happy (just look back on the arbitrage purge in AdWords many years ago if you don't believe that) With Search and AdWords, really the end customer is the searcher. With AdSense it is a little different, the end customer is the advertiser. If their advertisers are not happy with the sites they appear on through AdSense, and publishers don't take the hint... they will find a way to systematically wipe out anyone that does not fit in order to keep the end customer happy.