Robert_Charlton - 9:13 pm on Jan 21, 2012 (gmt 0)
I think that in the early iterations of Panda, Google was looking at an entire range of factors. They most probably decided that ads above the fold were one factor worth looking at in more detail. Perhaps there were ambiguities or uncertainties in the results they found.
This iteration is just about those above-the-fold factors. They've already decided about the brains a long time ago. They may then combine refinements from this iteration back into the overall algo, to see if the ads are still a deal breaker.
Google isn't just depending on its own preconceived notions, btw. It's testing those, and is looking at user behavior, and maybe trying to send some otherwise good sites a big hint about something that needs fixing... sort of like trying to tell someone that they have bad breath.
PS - It's very likely that the affected sites really don't have that much good content. If they did, why would they be hiding it?