Suggestions I have are:
Fade on pause and stop: It's a feature of Winamp when you use the Directsound output plugin
Also, would be cool if when you double clicked on the X in the taskbar it showed/hid Xion without pausing then playing the song currently playing. You click the X fast, Xion appears/hides, click it slow, it pauses.
Much like when you double click an Icon in windows it opens, but you do it too slow it goes into Rename state.