One thing that I think could really make Spotify better is a way to add notes or a user description of sorts to songs once they're added to a playlist. For example, I created a playlist called Trailer Tracks that has a number of songs featured in popular movie trailers. However, there is no way for me to tag each song with the trailer it was featured in.
A way of doing this, or allowing the admin of the playlist to add a description of why a certain song was added or what it means to them could really add a new twist on Spotify and take music sharing to a whole new level.
We're marking this as 'Not Right Now' as we don't have plans to implement this feature. Please continue to leave us your feedback around features you'd like to see though!