One thing I often do with my music library since it's so huge is add just about everything that I think is even remotely good to one big playlist. For example, the one I'm jamming to now has 678 songs in it because I just went through my Plex music library and added a ton of stuff one day.
However, I don't like it all, and I'd like to be able to, when I hear a song play in Plexamp that's on my playlist that I don't like, go in and "delete" that song quickly and easily, without having to open up a playlist management screen through my browser.
I realize this may require re-tooling the interface, or maybe building a whole new interface to which a user can toggle (e.g. leave the current interface as the default but add a button that when clicked brings up an interface management screen sort of like what you might see in Spotify); I would suggest that such an endeavor is one worthwhile.
This kind of feature could go pretty far, all the way to using the Plexamp client to handle all kinds of playlist management, including browsing the existing library (or multiple libraries?), adding to your playlist, and so on. I suggest a first pass, minimum feature MVP might be a button on the existing interface that just says, "Delete From Playlist" if indeed the song playing is on an actively used playlist.