Regarding the plugin system: They had a few different categories of plugins. You had input plugins that let it get audio from a wide variety of sources. A few I remember fondly were input plugins that were little emulators that ran playstation / n64 bytecode ripped from games to play their music. Then you could have your choice of DSP plugins applied after that, a visualization and then also an output. The output could play the music, but it could really do anything it wanted, like encoding it to ogg.

Another thing which makes a lot of sense but that I don't see in other music players is equalization. I want to believe that people nowadays handle that in another part of their audio chain, but realistically people are just forgoing it.