As much as I think mono is good for Linux, Python is more suited IMHO. There are more easily accessible libraries for Python and as such better plugin development is achievable by novice and expert.
Edit: just tried v.1.4.2 of Banshee, and it's okay...I prefer Rhythmbox
Banshee gripe: The app thinks it's a good idea to download all the cover art for all my music library in one go..all 7000+ mp3's of it. What's wrong with downloading as and when necessary