londrum - in my humble and surely non-comprehensive opinion, it seems to me that the previous algorithm heavily favored link metrics (as per the original PageRank), and when Amit Singhal came into the picture, he revamped the algorithm entirely. It took a long time and what we got was panda.
To your point, user engagement metrics have been around, sure, but implementation of them has never been as apparent. As an example is an option to "Block [domain.com]" when you immediately bounce back to a SERP. This is surely a harbinger of a new type of implementation.
I can't seem to locate the link where Amit pretty much states that he was given card blanche to change the algorithm, and he took a different approach than Larry Page did with his initial pagerank algo. If I find it I'll post it.