I'm using Spotify for almost a month now. Next to the many advantages I immediately notices one disadvantage. That is changing your own playlists order in the mobile app (iOS) or webplayer. This is really basic functionality and shouldn't be that hard to program in both platforms. Now I've to use the client app to modify the order. One of the benefits to use Spotify is that you can listen and modify your music everywhere. When adding music to my offline playlist the new songs are placed last, while I want to group them with other similar songs. So I've to wait untill I logon to the client app at home to modify the order. This is really frustrating when I'm on holidays, work and so on, and I don't have access to the client app.
Please consider this an improvement to the already great solution for music called Spotify.