It's the music discovery aspect of Spotify that is their game changer. They are constantly adding new ways to find new music.
That being said... I've been underwhelmed by the app's UI and performance and have given frequent feedback but haven't got any response and there has been no changes. Typical of Google... Its been so bad that I'm considering switching to Spotify soon. Its really annoying when you're trying to select some romantic music for dinner with your partner and the app keeps refusing to work.
Another thing I really enjoy about spotify is their app's ability to pause a song on my laptop at work and press play on my phone when I get in my car and the song picks up right where I left it. And the remote control functionality during parties is great. They're also experimenting with some "auto-DJ" features which don't do too bad of a job for an algorithm :)