Interesting. I've noticed on reccr ( http://reccr.com ), my own recommendation project, that the recommendations for 4 and 5 star ratings are much more accurate than those for 2 and 3 star ratings. I had initially thought that it was simply that there were more 4 and 5 star ratings in the system, and thus more data to base recommendations on, but the "larger perceived gap" between 2 and 3 versus 4 and 5 makes a lot of sense and is probably also a major contributing factor.