tedster - 2:14 am on Jul 2, 2011 (gmt 0)
I've been thinking about this idea for the past few hours, and it really holds water. If Google were trying to find shallow content - something that lies between spam and valuable content - one of the natural signs would be that the URL is already ranking for a decent query phrase. Any URL that isn't ranking for something strong would naturally be tossed out, because it's a waste of computational resources.
Then, as Panda evolves, the definition of "a decent query phrase" would slide further down into more long tail queries.
I find it curious that, according to your observation, results for some query terms now represent less diverse types of content. I'm going to ponder that some more, and also keep an eye out for any patterns like that,