@tedster That's a good point. The above the fold algorithm seems rather basic in comparison. Does somewhat fly in its face. Unless the previous implementation didn't look at above-the- fold specifically, but later decided to give that part extra weight. And yeah, not sure that this explains the idea that content is valued more. Unless it's a basic measure of x% of the page is taken up with actual content rather than ads + site template. Which isn't what I think of when I think of quality content, but it could be defined that way.
@kendo there are a lot of parts to design, some more subjective than others. As an extreme example, Google can fairly objectively state that if you stick black text on a black background, that will not be a good design outcome for their audience. Of course, you may be an artist and this could be some sort of intentional artistic vision. Remember, this is all about Google satisfying their searchers, not about directing people to the most artistically revolutionary site. The kinds of design choices that I think could be used in an algorithm are more related to the basic layout, font-sizes, and distribution of ads etc.
@Leosghost sorry I missed your post. I'm sure my thinking isn't original - there's so many theories it's impossible to keep track :) The site preview is not a bad way of getting some extra information about what kinds of layouts people like to see. At least on a basic level that gives some extra data points. And for competitive phrases it probably gives them quite a lot of useful extra information.