I am also very pleased with the Spotify feature, works like a charm!
There is one thing you probably already thought of and that is the possibility to use offline playlists. Are you working on an enhancement in the RuneUI to be able to download playlists and use it offline/toggle offline? When accessing the spotd daemon using telnet (telnet localhost 6602 on my RPi2) is seems spotd provides for offline use.
offline-status: display informations about the current status of the offline cache (number of offline playlists, sync status...).
offline-toggle pl: toggle offline mode for playlist number pl.
I must say I haven't tried it yet using telnet...