You may have already had this suggestion but I have used editing sw in the past that had this small feature and I seem to have become accustomed to it.
When you press the play button, it would be handy if it turned into a pause button and vice-versa.
I know the space bar will stop and start the mix but often I leave my hand on the mouse which is hovering over the play button and it would be nice to be able to just click again to stop the mix.
Just a thought,
Shane