So I have to say, frankly I'm hugely disappointed with the Windows Phone client. Considering that the Spotify experience over the years has become so great on iOS and Android, the Windows Phone app is inexcusably terrible.
The client misses basic features such as being able to fully manipulate playlists, the user interface is awful, the app is very slow to resume, pressing forward or backward too quickly from the volume panel just causes the music playback to crash, browsing the library is terrible (also no artist/album bio information, no way to sort tracks), the handover from 3G to Wi-Fi and reverse is slow and often just doesn't work (showing Offline instead), I've even had problems where the app won't let me play a playlist that I've synced offline because "no Internet connection is available". It's just frankly embarrassing.
I know you're going to say that the app is actually developed by Microsoft, and while that may be true, I would absolutely expect the Spotify team to be working with Microsoft at every available opportunity to improve this app.
I won't be renewing my Spotify Premium until someone starts to take this seriously - there's really no point if I can barely use it on my primary device.