Better if Space does Play/Pause/Resume rather than Play/Stop. GUI currently combines Play & Stop in 1 button and has a separate Pause/Resume button. Might be better to have one Play/Pause/Resume button and a separate Stop button. The Space bar would be equivalent of clicking Play/Pause/Resume button. Most decent MP3 player software (which excludes WMP) maps Space bar to Pause function.
Since there's a "Play Next Song" button, why not a "Play Previous Song" button?
Must think about this. I would be reluctant to make TOO many changes to the behavior of the Play/Stop button as people have gotten used to the current behavior.
"Play Previous Song" was left out mostly due to laziness.