Why is it so good? Well, you can listen to whatever you want, create playlists and even cache songs to listen to while you're not connected, but most importantly, the UI is clean and easy to use making the user experience great. Spotify has a good visual layout; screen density is low and navigation is inituitive and flexible helping users find music quickly. On the left side, for example, the Search box, recently saved searches and the album cover link each represent important navigation elements.
Also, the Search box and 'New Playlist' link are very visible and prominent functionalities for users with those specific goals in mind. Although using the Radio filter is cumbersome (later post), overall, Spotify is nice and I'm really looking forward to seeing it on mobile devices soon.