That's an excellent post by Hissingsid and well worth pursuing.
Rather than wanting to make the web better in general, I think Google really wants only to be able to select the best sites (from a user perspective) for their SERPS. They still need rubbish sites on the web because if all web pages were good then all search engines would have good SERPS.
In essence they want their SERPS to be the best and the other SERPS mediocre or bad for users.
Agreed, whatever criteria Google are using they are changing it constantly to stop SEO people from understanding the system. They are doing this on purpose and it's working a treat.
Another criteria they might be using is how often a brand/site name is used when users enter a search. E.G. if users frequently enter "buy widgets Amazon" then Google judge that Amazon is a good website for widgets. This would go some way to explaining why big brands appear to be doing well post Panda.