To avoid running up data charges for those of use without unlimited plans on our mobile devices:
It would be fantastic if spotify could detect the absence of the WiFi network and automatically switch to offline mode to avoid streaming on 3G/4G. Ideally, you could turn this option on or off depending on your preference. Even the ability to simply turn off 3G streaming would be an improvement (this option already exists for synching).
As it is, i have to remember to turn offline mode on and off everytime i leave a wifi network. This frequent toggling of the offline mode has resulted in the loss of my synched content on several occasions. Adding this feature to the settings menu would save me time and money, improve the overall experience of the app, and create another draw for the premium subscription.