Vanessa Fox did the leg work to get Google to provide details on this update and posted her findings at Search Engine Land. In short, Google confirmed the update twice, once at the Google I/O, as we said and once again to us at Search Engine Land.
Matt Cutts of Google said at Google I/O and confirmed in an email to us that he did say this:
This is an algorithmic change in Google, looking for higher quality sites to surface for long tail queries. It went through vigorous testing and isn’t going to be rolled back.
In a follow up with Google's team, Google said this was a change to the rankings algorithm and not a change to the indexing or crawling areas.
This does impact long-tail keywords, as covered almost a month ago. It is also Google is confident and said they won't roll back. Vanessa's expert opinion was:
This change seems to have primarily impacted very large sites with “item” pages that don’t have many individual links into them, might be several clicks from the home page, and may not have substantial unique and value-added content on them. For instance, ecommerce sites often have this structure. The individual product pages are unlikely to attract external links and the majority of the content may be imported from a manufacturer database. Of course, as with any change that results in a traffic hit for some sites, other sites experience the opposite. Based on Matt’s comment at Google I/O, the pages that are now ranking well for these long tail queries are from “higher quality” sites (or perhaps are “higher quality” pages).
My complete speculation is that perhaps the relevance algorithms have been tweaked a bit. Before, pages that didn’t have high quality signals might still rank well if they had high relevance signals. And perhaps now, those high relevance signals don’t have as much weight in ranking if the page doesn’t have the right quality signals.
The folks over at WebmasterWorld are now picking apart this new evidence and trying to isolate which niches these might impact specifically and what can be done to reverse the impact.
I should note that when analyzing this, make sure your traffic didn't decline due to the redesign. We ran a poll and 43% said traffic dropped due specifically to the redesign at Google and not MayDay.
Update: Matt Cutts of Google posted a video on this topic, watch the May Day Video on our blog.