In Windows 8 (8.1 specifically) the play/pause button on my Sennheiser Bluetooth A2DP headphones does not play or pause Spotify. The track skip buttons work, but not the play/pause button.
Windows 8 supports AVRCP as shown by the working skip buttons, and indeed all buttons work with Windows Media Player; just not the play/pause button with Spotify.
I'm at a loss of where to look to even diagnose this, as Spotify isn't like Winamp - its a bit of a black-box with its configuration and logging.
Perhaps this is just a Spotify bug; I will see if I can find some direct-line to support in parallel.