I've been trying to watch some of Amazon's free Prime video content, but it's very difficult - I have to wade through page after page of My Little Pony to get to anything I'm interested in. There is practically no discoverability features besides "customers who watched this also...", which is generally not very useful.
Moving ratings? They are so highly variable for media, and so subjective to personal taste, that they are practically useless. Global reviews work fine for electronics, but they fall apart utterly when applied to content. Go look at the star rating distribution for any reasonably controversial book - all over the place, and tells me nothing about how I would like it.
The MP3 store is similar. iTunes does a pretty decent job with Genius, but is still pretty poor discoverability-wise. Spotify's new-content discoverability is also extremely poor. Rdio is pretty good, but I would pay for a Pandora that plugs into a music store/subscription service. IMO this is some pretty lazy engineering - what works for selling DVD players does not work for selling content, but Amazon's UI for browsing digital video is practically identical to their physical-shopping UI, and that's entirely unacceptable.