On the homescreen widget, when the album art is pressed the app is launched. It would be nice to have the same functionality with the lockscreen widget because it would speed up the process of changing what I'm listening to or adding more music to the playlist.
The other enhancement I would like to see is the lockscreen widget to load automagically when something is plugged into the headphone jack. I saw another feature request for automatically starting the music, but this may not be desired in all situations. if the lockscreen widget is loaded instead, you could start the playback without unlocking the screen which would be very fast without doing something the user didn't explicitly for.