Logging this as a bug, because it's out of character. I have used Spotify for a long time now on iPhone before switching to WP8, and I never once observed this kind of behavior:
Data usage is excessive.
After just 2 hours of using spotify, I was hit with a 1.2GB+ data bill by my carrier. That is to say that in just this period, Spotify pulled down over 1.2GB of data streaming music - and there may be more.
I have never seen the iPhone version come even close to this kind of level of traffic - I would routinely use it on my trips to and from work, streaming new music I didn't have cached, and the highest I can ever recall seeing in one day was around 150MB.
Unfortunately this has already meant massive data charges for me - a single days use costing more than multiple months subscription to Spotify, which I paid for three months while they didn't even have a Windows Phone 8 app available which I could use!
I expect to not be the only one caught out by this.
Handset: Nokia Lumia 920
OS Version: 8.0.10211.204
Firmware revision number: 1232.5951.1249.1007
Hardware revision number: 22.214.171.124
Using LTE and 3G (Telstra).