I believe that there are two other factors that could be playing a part in “brand preference” is :

1. Traffic + average user time on site
A well known brand is likely to attract more traffic due to the extent of their marketing in other media, brand loyalty and consumer awareness. Alexa comparison of top ranking sites???

A reader is likely to spend more time on a “brand” website where the information is trusted rather than on a generic site where they tend to scan content and click away usually within 30 seconds.

2. Direct address access vs links from a search engine
Surfing trends show that often internet users will attempt to ‘guess’ a URL (often with some success) rather than resorting to a search engine as they have recently grown frustrated with the onslaught of useless links in the SERP’s. Addresses that have been bookmarked or accessed directly may tend to perform better than links from SERP’s.

The above two points are trends that we have been noticing on the SERP’s for health and medical content just prior to the December 2008 update.

Matt Cutt’s may not be evading the question when he offers the answer “make great content” in response to enquiries about upcoming SEO changes. Brand websites may not always be laden with text content but have you noticed how their flash videos, online forms and calculators or little scripts that offer a level of user interactivity tend to keep us glued onto the site for longer than 30 seconds?

It’s a collection of factors, never just one and Google just loves keeping us SEO’s on our toes. Looking forward to hearing more about your findings Matt.

Once again, great post on a relevant topic – always useful information.