This month, there are already a lot of different observations from the WebmasterWorld discussion on Google SERP changes. Forum members have noted that Google is looking for more fresh results and is specifically weeding out old and outdated pages. One forum member notes the following:
I think Google looks at a site and compares all the content, then keeps some of the most recent content in the results including the higher PR stuff and puts the older stuff in supplemental. That is only a guess but seems to be what is happening.
Additionally, reciprocal linking has been touched upon. It looks like Google is paying more attention to artificial linking versus natural linking schemes. From personal observations as well, I believe this to also be the case.
Numerous forum members are reporting traffic drops and ranking fluctuations as well.
I'm sure there will be a lot more discussion, so check out the thread at WebmasterWorld for additional updates.