If Google's music streaming service goes something like this, I think it will cause great pain to services like Spotify/Rdio...
1. Have a filter to catch song titles/lyrics from a Google search (something like searching for locations pulling up Google Maps cards)
2. Have a quick play action/add to playlist
3. Integrate into YouTube, same as they do now for iTunes and AmazonMP3 links (Vevo may have trouble here)
4. Subscriptions starting at the same (or less) than Spotify's rates, and payable through Google Play Credit
5. Clients for all (Android/iOS/Chrome)
If it can do these things, I think it will be highly successful. They basically just need to emulate/buy Spotify, whilst integrating YouTube and Search to corner the market. #IO13