I find it interesting that there seems to be an asymptotic increase in difficulty right around the 10 percent mark. Did Netflix estimate this before they created the Prize, or did they think it would be easier (or harder) to get there--perhaps that they would have gotten there already?
There has to a be a limit to the amount of data that can be taken from what is provided to improve the effectiveness of the suggestions, I'd say there close to the point in which techniques to make sense of the data have reached there limit.
The system can only be effective to a point because of the unpredictability of a user.
