dynamic systray icon
This perhaps one of the most visible issues (for me) in day-to-day usage.
Currently Audacious uses a static icon in the systray, whether it is in Play, Pause or Stop mode. It happens often that I'm confused if something is currently playing (say, silent passage at start / end of track) and I end up doing several rounds of Play / Pause until I figure out what's going on. This is especially true when Audacious is closed to systray.
By comparison, when playing Sonata uses:
and when stopped, the standard icon:
It would be very useful if Audacious included similar icons and displayed them dynamically depending on its current state.