that Google is isolating some factors within "quality" and testing them separately
It could be. We've long shared the view Panda would evolve from being in an infant state.
So potentially, what we might be seeing is certain elements being dropped from the main iteration, "so called" Panda updates , since they will update more or less dynamically, and gradually these Panda updates will become less significant in terms of the overall quality score management.
The speed of indexing of these new pages with new content that is added is the best I've ever seen on these sites.
As a further observation, i did also notice pages on sites , that i would term " more heavily pandalized " or in the early stages of recovery works, show very limited life. The response characteristics are mainly ; slow to index , impossible to rank new pages in isolation - but Google is abundantly aware of changes going on as they start to build.
@Tedster - did you take those comments by Amit Singhal to mean that this element that I'm describing was already in play ? I might be behind the times - but I've seen nothing mentioned elsewhere.