Deep Neural Networks for YouTube Recommendations

Recommender systems moved from explicit feedback (like ratings) to implicit feedback precisely because users are less likely to actually rate stuff and also because ratings are subjective; by which I mean your interpretation of 3 stars(good) may not agree with mine(average). I have watched tons of movies/shows on netflix or videos on YT for that matter but have not rated a single video. To address the other part of your suggestion i.e collapse ratings/feedback across media like movies,books,etc. usually it is very difficult to have a dataset that spans multiple media across the same set of users. Even if it is present it would be too sparse (more sparse than usual for a site like YT with a continuously changing content library) to actually help. Though I agree that if anybody can get the recommender right, it is Google with the sheer amount of info it has on each user.