One thing I was talking to checker about the other day is the way user's ratings are highly affected by the rating that was predicted for them. He claimed this makes recommender systems broken. I contend that the recommender is working totally fine, the problem is the Web 2.0 web site that let you see the guessed rating before you rate yourself.
There are a few different psychological factors at work. One is that the guessed rating becomes a baseline. Then if you like it you want to rate above that baseline, and if you didn't you rate below it, so you're not really using the absolute scale. The other is that seeing the rating before you watch the movie preconditions your response; it tells you something is supposed to be really great and you are biased based on that.
Did I never write a description of my ideas on Netflix? I can't find it anywhere in my old rants. I should do that.