Why, god, why bother?
I haven't figured out the point.
"10 million tracks" is not a lot in terms of absolute catalogue. It'll be all the same (popular) content as every other MP3 store, only less of it, as their deals settle in to line. 7digital and iTunes, and even Amazon MP3, have way way more - and still don't cater effectively to the long tail, who has to go to smaller stores such as Beatport or DJDownload to get what they need.
Even then, there's really no point without an effective referral scheme and integration with music discovery websites such as Last.fm or Spotify, where people go to FIND new music.
HMV should have done this ten years ago, or invested in someone who knows what they're doing, like 7digital.